Microorganism genomics, compositions and methods related thereto

ABSTRACT

The present invention provides methods and composition for accessing, in a generally unbaised manner, a diverse genetic pool for genes involved in biosynthetic pathways. The invention also provides compounds which can be identified by cloning biosynthetic pathways.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.08/969,651 filed Nov. 13, 1997, now U.S. Pat. No. 6,261,842 which is acontinuation-in-part of U.S. patent application Ser. No. 08/956,692filed Oct. 24, 1997, now abandoned, which claims the benefit of U.S.Provisional Application Ser. No. 60/063,230 filed Oct. 23, 1997.

GOVERNMENT FUNDING

Work described herein was supported by funding from the NationalInstitutes of Health. The United States Government has certain rights inthe invention.

BACKGROUND OF THE INVENTION

Until recently, it was assumed that cultivation of microorganisms fromthe environment resulted in the isolation of a good proportion of themicroorganisms present. Phylogenetic analysis of rRNA sequences obtainedfrom direct sampling of environments has shown that this is not thecase. Giovannoni et al. (1990) Nature 345:60–63; Pace et al. (1996) ASMNews 62:463–470; Stahl et al. (1985) Appl. Environ. Microbiol.49:1379–1384; Suzuki et al. (1997) Appl. Environ. Microbiol. 63:983–989;Ward et al. (1990) Nature 345:63–65. It is now apparent that themicroorganisms that can be cultured from any environment using standardtechniques probably represent the minority of the total species presentin that environment, indicating that a vastly greater diversity ofprokaryotes exists than suggested by culturing methods. Pace et al,supra; Stahl (1993) ASM News 59:609–613. The idea that perhaps the vastmajority of bacteria in an environment are currently nonculturable hasrevolutionized thinking in microbiology, and has stimulated newapproaches to the study of microbes. Woese et al. (1990) PNAS87:4576–4579.

For instance, it is estimated that the number of species currentlyculturable from soil represents 1% or less of the total population.Griffiths et al. (1996) Microbial Ecol. 31:269–280; Torsvik et al.(1996) J. Ind. Microbiol. 17:170–178. DNA—DNA reassociation measurementshave been used to determine total genetic diversity in one soil sample.The data indicated that greater than 4000 species might be present.Torsvik et al. (1990) Appl. Environ. Microbiol. 56:782–787. Thisrepresented at least 200 times more diversity than was observed byexamining culturable bacteria from the same sample. Another study basedon methods that did not involve culturing suggested 13,000 species in100 g soil. Torsvik et al. (1994) p. 39–48, In Beyond the Biomass, K.Ritz, J. Dighton and K. E. Giller (eds.), John Wiley and Sons,Chichester. By estimating the total number of cells at 5×10¹¹ per gramof soil, this suggested an average of 5×10⁷ cells per species assumingeven species distribution. Thus even rare species might have fairlylarge population sizes in the soil. A recent analysis in our labsindicated that novel phyla of Bacteria and Archaea are present in soil.Bintrim et al. (1997) PNAS 94:277–282; Bintrim et al., in press. Of 144cloned Bacterial 16S rRNA gene sequences examined, 45 had the closestaffiliation to members of the phylum Proteobacteria, but of theseclones, only 6 had close affiliation to known genera (Pseudomonas,Hafnia, Azospirillum). The clones were distributed across the entireDomain, and none were identical to any previously known sequence.Moreover, these studies revealed entirely new lineages of microbes insoil, both from the Domain Archaea and the Domain Bacteria. Thisindicates the enormous diversity of noncultured microorganisms fromsoil.

SUMMARY OF THE INVENTION

The present invention, in one aspect, provides methods and reagents foridentifying genes from microbial organisms, the gene products of whichare involved in biochemical transformation reactions that produce, forexample, small organic molecules by de novo synthesis, or thatchemically modify molecules ectopically provided in the microbe'senvironment. In general, the method provides host cells which have beenengineered to express the opening reading frames of genomic DNAsub-cloned from a heterologous microorganism. The subject method detectschanges in the phenotype of the host cell which are dependent onexpression of open reading frames from the genomic DNA, e.g., which maybe marked by altered biosynthetic capabilities.

Another aspect of the invention provides methods and reagents foridentifying biosynthetic products, preferably other than those producedby ribosomal synthesis, which are generated by recapitulation of aheterologous microbial biosynthesis pathway in a host cell, or generatedby a chimeric metabolic pathway involving both heterologous andendogenous gene products in the host cell. As above, the assay generallydetects biochemical transformation reactions that produce, for example,small organic molecules by de novo synthesis, or that chemically modifymolecules ectopically provided in the host microbe's environment. Ingeneral, the method provides host cells which have been engineered toexpress the opening reading frames of genomic DNA sub-cloned from aheterologous microorganism. Likewise, this embodiment of the subjectmethod can be disposed to detect changes in the phenotype of the hostcell which are dependent on the formation of a natural product, or thetransformation of an ectopically added agent.

Thus, for example, there is provided a method for identifying a productof a biosynthetic pathway, comprising

-   -   i) providing host cells containing a replicable vector including        genomic DNA isolated from a source of uncultivated        microrganisms, which host cells are provided under conditions        wherein expression of open reading frame sequence(s) of the        genomic DNA occurs; and    -   ii) detecting a compound produced by the host cells, e.g.,        relative to host cells lacking the genomic DNA.

In another embodiment, the present invention provides a method forcloning genes of a biosynthetic pathway, comprising

-   -   i) providing host cells containing a replicable vector including        genomic DNA isolated from a source of uncultivated        microrganisms, which host cells are provided under conditions        wherein expression of open reading frame sequence(s) of the        genomic DNA occurs; and    -   ii) detecting the presence or absence of a biosynthetic pathway        which is dependent on expression of at least one of the opening        reading frames by the host microorganisms.

There is also provided a method for cloning genes of a biosyntheticpathway, comprising

-   -   i) cloning, into a replicable vector, genomic DNA from a source        of uncultivated microrganisms;    -   ii) expressing open reading frame sequence(s) of the genomic DNA        in a host microorganism harboring the vector; and    -   iii) detecting the presence or absence of a biosynthetic pathway        which is dependent on expression of at least two of the opening        reading frames by the host microorganism.

That method can also be used to identify a product of such abiosynthetic pathway produced in the host microorganism. In preferredembodiments, the biosynthetic pathway produces or transmutes anon-polymeric and/or non-proteinaceous compound. In certain preferredembodiments, the biosynthetic pathway produces a product having amolecular weight less than 7500 amu, more preferably less than 5000 amu,and even more preferably less than 2000 amu.

Yet another aspect provides a method for detecting a non-proteinaceouscompound produced by a microorganism, comprising

-   -   i) sub-cloning, into a replicable vector, genomic DNA from one        or more uncultivated microrganisms;    -   ii) expressing open reading frame sequence(s) of the genomic DNA        in a host microorganism harboring the vector;    -   iii) detecting ectopic production of a non-proteinaceous        compound by the host microorganism.

In other embodiments, there is provided a method for cloning two or moregenes encoding gene products functioning in a biological pathway of amicroorganism, comprising

-   -   i) sub-cloning, into a replicable vector, genomic DNA from one        or more uncultivated microrganisms;    -   ii) expressing at least two open reading frame (ORF) sequences        of the cloned genomic DNA in a cultivable host microorganism        transfected with the vector;    -   iii) identifying ORF sequences which confer a phenotypic change        on the host microorganism, which phenotypic change is dependent        on the expression of at least two ORF sequences of the cloned        genomic DNA.

In still other embodiments, there is provided a method for cloning genesencoding gene products functioning in the chemical transformation of anon-proteinaceous compound by a microorganism, comprising

-   -   i) sub-cloning, into a replicable vector, genomic DNA from an        uncultivated microrganisms;    -   ii) expressing open reading frame (ORF) sequence(s) of the        cloned genomic DNA in a cultivable host microorganism        transfected with the vector;    -   iii) detecting a phenotypic change of the host cell, which        phenotypic change is dependent on the expression of at least two        ORF sequences of the cloned genomic DNA.    -   iii) identifying one or more ORF sequence which confer a        phenotypic change on the host cell, which phenotypic change is        dependent on the chemical transformation of the        non-proteinaceous compound.

In yet another embodiment, there is provided a method for identifyingnon-proteinaceous compounds produced by a uncultivated microorganisms,comprising

-   -   i) generating a library of host microorganism transfected with a        variegated population of vectors containing genomic DNA isolated        from a sample of uncultivated microrganisms, which genomic DNA        includes open reading frame (ORF) sequences which can be        expressed from the vector in the host microorganism;    -   ii) culturing the transfected host microorganism under        conditions wherein the ORFs are expressed;    -   iii) detecting ectopic production of non-proteinaceous compounds        by the host microorganisms.

In preferred embodiments of the methods of the present invention,uncultivated mircoorganisms are prokaryotes. For instance, themircoorganism can be archaea microorganism, such as Crenarachaeota,Euryarchaeota, or Korachaeota.

The mircoorganism(s) can be isolated from such sources as soil, insectintestines, plant rhizospheres, microbial mats, sulfur pools, marinesamples and the like. One source of the microorganism is soil. Anothersource is environments of extreme pH (e.g., less than 1 or greater than12) or temperature (e.g., greater than 80° C., or even greater than 100°C.).

In preferred embodiments, the sub-cloned genomic DNA is at least 25, 50,75 or 100 kilobases in length.

The variegated population of vectors preferably include sub-clonedgenomic DNA from at least 10, 10³, 10⁴ or even 10⁵ differentmicroorganism species.

In preferred embodiments, the host microorganism is a species from theBacteria, such as may be selected from the group consisting ofAcetobacter, Actinomyces, Aerobacter, Agrobacterium, Azotobacter,Bacillus, Bacteroides, Bordetella, Brucella, Chlamydia, Clostridium,Corynebacterium, Erysipelothrix, Escherichia, Francisella,Fusobacterium, Haemophilus, Klebsiella, Lactobacillus, Listeria,Mycobacterium, Myxococcus, Neisseria, Nocardia, Pasteurella, Proteus,Pseudomonas, Rhizobium, Rickettsia, Salmonella, Serratia, Shigella,Spirilla, Spirillum, Staphylococcus, Streptococcus, Streptomyces,Treponema, Vibrio, and Yersinia. Escherichia and Streptomyces are mostpreferred.

In preferred embodiments, the vector is a low-copy number vector, suchas a single-copy number vector.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1: pBeloBAC11 vector

FIG. 2: Is a table illustrating advantages of utilizing BAC vectorsystems.

FIG. 3: is a table illustrating the average size inserts in various BAClibraries described in the art.

FIG. 4: compares the average insert for various library types.

FIG. 5: is a table describing the phenotypes confered on the host cellby the expression of the Bacillus cereus BAC library.

FIG. 6: is a table outlining the heterologous expression of naturalproducts pathways in Streptomyces species.

DETAILED DESCRIPTION OF THE INVENTION

I. General Overview

Traditional methods of natural product discovery have relied onculturing microbes from the environment and implementing screens to testwhether these cultured strains produce metabolites of interest. Francoet al. (1991) Crit. Rev. Biotech. 11:193–276. This has been a remarkablysuccessful approach, but new detection methods as well as new sourceorganisms are needed.

The present invention, in one aspect, provides methods and reagents foridentifying genes from microbial organisms, the gene products of whichare involved in biochemical transformation reactions that produce, forexample, small organic molecules by de novo synthesis, or thatchemically modify molecules ectopically provided in the microbe'senvironment. In general, the method provides host cells which have beenengineered to express the opening reading frames of genomic DNAsub-cloned from a population of heterologous (“source”) microorganisms.In general, the method begins with a variegated population of host cellsengineered with the sub-cloned DNA, which in turn was isolated in amanner which greatly increases the complexity of the library byincluding genomes of hitherto unaccessed species of microorganisms. Thesubject method detects altered biosynthetic capabilities of theengineered host cells resulting from expression of open reading framesin the heterologous genomic DNA. The subject method takes a functionalapproach to screening the genomic libraries, requiring that theexpression of the cloned genomic DNA recapitulates a biosyntheticpathway from the source organism, or combines with the gene products ofthe host organism to form a new chimeric pathway. This provides canprovide a rapid and efficient means for cloning new genes of significantinterest and identifying new biosynthetic products produced therefrom.

In samples from almost any environmental source, including those fromextreme environments, one can generally find a widely diverse populationof microorganisms, However, the microorganisms which are isolated bymost standard culturing techniques are thought to represent only a tinyfraction of the total microorganisms in any environment. By eliminatinginitial culturing steps in the sub-cloning process, one of the salientfeatures of the subject method is that it can be carried out in a mannerwhich provides a relatively unbaised approach to cloning components ofbiosynthetic pathways. In this regard, the method can directly accessthe genetic material of a complex sample of microorganisms in a mannerwhich can better preserve the phylogenetic diversity of themicroorganism population in the subcloned DNA. Moreover, genomicsequences can be collected from microorganisms which exist only underextreme conditions, such as extreme temperatures or extreme pH's. Thiscan greatly enhance the likelihood that novel biosynthetic pathways andtheir products can be identified.

For instance, one embodiment the subject method provides a means forcloning genes, preferably sets of two or more genes, whose expressionproducts recapitulate a biosynthetic pathway of the source organism, orcreate chimeric biosynthetic pathways in a host cell. In general, themethod includes a step of directly sub-cloning, into a replicablevector, genomic DNA isolated directly from an uncultivated sample ofmicrorganism(s). A cultivable host microorganism is transfected with theresulting variegated population of vectors, and the transfected hostcells are cultured under conditions wherein open reading frame (ORF)sequences of the cloned genomic DNA are expressed in the host cell. Thegeneration of a new biosynthetic pathway in the host organism can bedetected by any of a number of techniques involving, for example,chemical, photometric, biochemical and/or biological assay techniquesfor natural products. In this manner, the DNA of microorganisms whichare difficult to culture, or are unculturable by current techniques, isnow accessible and amenable to propagation and expression in organismsthat are more easily cultured. Ths, such genetic information can bebetter represented in a functional genomics approach to identifyingnovel biosynthetic pathways.

Another aspect of the invention relates to the identification of thebiosynthetic products, e.g., other than those produced by directribosomal synthesis, which can be produced by the recapitulation ofheterologous biosynthetic pathways as describe above. As above, theassays for many natural producs are generally derived to detectbiochemical transformation reactions that produce, for example, smallorganic molecules by de novo synthesis, or that chemically modifymolecules ectopically provided in the host microbe's environment. Ingeneral, the method provides host cells which have been engineered toexpress the opening reading frames of genomic DNA sub-cloned from aheterologous polulation of microorganism, e.g., from a naturalassemblage. As above, formation of a natural product, or thetransformation of an ectopically added agent can be detected by assaysutilizing chemical, photometric, biochemical and/or biologoicaldetection techniques.

It is not expected that every pathway provided in a library of clonedgenomic DNA will recapitulate a functional pathway in the host cell;however, even if only a small number of the cloned pathways areexpressed, the probability of discovering novel compounds remains high.To illustrate, a BAC library of 500,000 clones, each with an insert ofat least 100 Kb, could include the genomes of 10,000 differentmicroorganisms, given an average genome size of 5 Mb. This represents anenormous amount of genetic material available for analysis. Even if thefrequency of heterologous expression of a pathway leading to a bioactiveproduct is only in the range of 0.1 to 1%, there will be 500 to 5,000clones with such an activity in a library of this size. This estimate isbelieved to be conservative given that at least 20% of culturable soilmicroorganisms produce antimicrobial metabolites and a reasonableproportion of genes from diverse microbes can be expressed in the systemof the present invention.

Furthermore, the practice of the subject method can contribute to thebasic understanding of microbial populations in nature. Currentinformation on noncultured microbial communities is almost exclusivelyof a phylogenetic nature. While this is extremely powerful and importantinformation, it does not provide a good measure of the physiologicalpotential of these populations, as phylogeny is not a complete indicatorof physiological diversity or metabolic capacity (Stahl et al. (1993)ASM News 59:609–613; and Stein et al. (1996) J. Bacteriol. 178:591–599).Thus, use of the subject approach of analyzing the physiologicaldiversity of noncultured microbes can make significant contribution tobasic research on microbial communities, which in turn has directimplications for understanding evolution and spread of infectious agentsand antibiotic resistance genes.

II. Definitions

As used herein, the term “microorganism” includes prokaryotic andeukaryotic microbial species from the Domains Archaea, Bacteria andEucarya, the latter including yeast and filamentous fungi, protozoa,algae, or higher Protista. The terms “microbial cells” and “microbes”are used interchangeably with the term microorganism.

The term “prokaryotes” is art recognized and refers to cells whichcontain no nucleus or other cell organelles. The prokaryotes aregenerally classified in one of two domains, the Bacteria and theArchaea. The definitive difference between organisms of the Archaea andBacteria domains is based on fundamental differences in the nucleotidebase sequence in the 16S ribosomal RNA.

The term “Archaea” refers to a categorization of organisms of thedivision Mendosicutes, typically found in unusual environments anddistinguished from the rest of the procaryotes by several criteria,including the number of ribosomal proteins and the lack of muramic acidin cell walls. On the basis of ssrRNA analysis, the Archaea consist oftwo phylogenetically-distinct groups: Crenarchaeota and Euryarchaeota.On the basis of their physiology, the Archaea can be organized intothree types: methanogens (prokaryotes that produce methane); extremehalophiles (prokaryotes that live at very high concentrations of salt([NaCl]); and extreme (hyper) thermophilus (prokaryotes that live atvery high temperatures). Besides the unifying archaeal features thatdistinguish them from Bacteria (i.e., no murein in cell wall,ester-linked membrane lipids, etc.), these prokaryotes exhibit uniquestructural or biochemical attributes which adapt them to theirparticular habitats. The Crenarchaeota consists mainly ofhyperthermophilic sulfur-dependent prokaryotes and the Euryarchaeotacontains the methanogens and extreme halophiles.

“Bacteria”, or “Eubacteria”, refers to a domain of prokaryoticorganisms. Bacteria include at least 11 distinct groups as follows: (1)Gram-positive (gram+) bacteria, of which there are two majorsubdivisions: (1) high G+C group (Actinomycetes, Mycobacteria,Micrococcus, others) (2) low G+C group (Bacillus, Clostridia,Lactobacillus, Staphylococci, Streptococci, Mycoplasmas); (2)Proteobacteria, e.g., Purple photosynthetic +non-photosyntheticGram-negative bacteria (includes most “common” Gram-negative bacteria);(3) Cyanobacteria, e.g., oxygenic phototrophs; (4) Spirochetes andrelated species; (5) Planctomyces; (6) Bacteroides, Flavobacteria; (7)Chlamydia; (8) Green sulfur bacteria; (9) Green non-sulfur bacteria(also anaerobic phototrophs); (10) Radioresistant micrococci andrelatives; (11) Thermotoga and Thermosipho thermophiles.

“Gram-negative bacteria” include cocci, nonenteric rods, and entericrods. The genera of Gram-negative bacteria include, for example,Neisseria, Spirillum, Pasteurella, Brucella, Yersinia, Francisella,Haemophilus, Bordetella, Escherichia, Salmonella, Shigella, Klebsiella,Proteus, Vibrio, Pseudomonas, Bacteroides, Acetobacter, Aerobacter,Agrobacterium, Azotobacter, Spirilla, Serratia, Vibrio, Rhizobium,Chlamydia, Rickettsia, Treponema, and Fusobacterium.

The term “pathogen” is art recognized and refers generally to anyorganism which causes a deleterious effect on a selected host underappropriate conditions. Within the scope of this invention the termpathogen is intended to include fungi, bacteria, nematodes, viruses,viroids and insects.

“Gram positive bacteria” include cocci, nonsporulating rods, andsporulating rods. The genera of gram positive bacteria include, forexample, Actinomyces, Bacillus, Clostridium, Corynebacterium,Erysipelothrix, Lactobacillus, Listeria, Mycobacterium, Myxococcus,Nocardia, Staphylococcus, Streptococcus, and Streptomyces.

As used herein, the term “nucleic acid” refers to polynucleotides suchas deoxyribonucleic acid (DNA), and, where appropriate, ribonucleic acid(RNA). The term should also be understood to include, as equivalents,analogs of either RNA or DNA made from nucleotide analogs, and, asapplicable to the embodiment being described, single (sense orantisense) and double-stranded polynucleotides.

The term “kb” refers to kilobases, e.g., thousands of contiguousnucleotide bases.

As used herein, the terms “gene” and “recombinant gene” refer to anucleic acid sequence which is transcribed and (optionally) translated.Thus, a recombinant gene can comprise an open reading frame encoding apolypeptide. In such instances, the sequence encoding the polypeptidemay also be referred to as an “open reading frame”. In otherembodiments, a gene can simply provide, on transcription, an antisensetranscript, a ribozyme, or other RNA molecule which effects thephenotype of the host cell.

“Transfection”, as used herein, refers to the insertion of an exogenouspolynucleotide into a host cell, irrespective of the method used for theinsertion, for example, direct uptake, transduction, mating orelectroporation.

The term “expression” with respect to a gene sequence refers totranscription of the gene and, as appropriate, translation of theresulting mRNA transcript to a protein. Thus, as will be clear from thecontext, expression of a protein results from transcription andtranslation of the open reading frame sequence. On the other hand,“expression” of an antisense sequence or ribozyme will be understood torefer to the transcription of the recombinant gene sequence.

“Transcriptional regulatory sequence” is a generic term used throughoutthe specification to refer to DNA sequences, such as initiation signals,enhancers, and promoters, which induce or control transcription of agene or genes with which they are operably linked.

By “operably linked” is meant that a gene and transcriptional regulatorysequence(s) are connected in such a way as to permit expression of thegene in a manner dependent upon factors interacting with the regulatorysequence(s).

The terms “host cells” and “recombinant host cells” are usedinterchangeably herein. It is understood that such terms refer not onlyto the particular subject cell but to the progeny or potential progenyof such a cell. Because certain modifications may occur in succeedinggenerations due to either mutation or environmental influences, suchprogeny may not, in fact, be identical to the parent cell, but are stillincluded within the scope of the term as used herein.

The term “PAC” is art recognized and refers to P1 artificialchromosomes.

The term “BAC” is art recognized and refers to bacterial artificialchromosomes

As used herein, a “reporter gene” is a gene whose expression may beassayed; reporter genes may encode any protein that provides aphenotypic marker, for example: a protein that is necessary for cellgrowth or a toxic protein leading to cell death, e.g., a protein whichconfers antibiotic resistance or complements an auxotrophic phenotype; aprotein detectable by a colorimetric/fluorometric assay leading to thepresence or absence of color/fluorescence; or a protein providing asurface antigen for which specific antibodies/ligands are available.

The term “biosynthetic pathway”, also refered to as “metabolic pathway”,refers to a set of anabolic or catabolic biochemical reactions forconverting (transmutting) one chemical species into another. Forinstance, an antibiotic biosynthetic pathway refers to the set ofbiochemical reactions which convert primary metabolites to antibioticintermediates and then to antibiotics.

The term “non-ribsomal synthesis” refers to a biosynthetic step orseries of steps other than peptide bond formation in the translation ofmRNAs into polypeptides. That is, the term refers to biosynthetic stepsother than peptidyl transferase-catalyzed formation of peptide bonds.Likewise, “transformation of a non-proteinaceous compound” refers to thebiochemical modification of a compound which is not directly produced byribosome-mediated formation of peptide bonds

“Ribosomal peptide synthesis”, on the other hand, refers toribosome-mediated formation of peptide bonds in the synthesis ofpolypeptide; though it does not include post-translational modificationof the polypeptide by ribosome-independent reactions.

A “non-proteinaceous compound” refers to a compound which not producedby ribosome-mediated formation of peptide bonds. Thus the term includesthe macrolide class of compounds and the like.

A “small molecule” refers to a compound which is not itself the productof gene transcription or translation (protein, RNA or DNA). Preferably a“small molecule” is a low molecular weight compound, e.g., less than7500 amu, more preferably less 5000 amu and even more preferably lessthan 2500 amu. Examples of small molecules include, among the manycompounds commonly referred to as “natural products”, beta-lactamantibiotics, steroids, retinoids, polyketides, etc.

“Peptide antibiotics” are classifiable into two groups: (1) those whichare synthesized by enzyme systems without the participation of theribosomal apparatus, and (2) those which require theribosomally-mediated translation of an mRNA to provide the precursor ofthe antibiotic.

The “non-ribosomal peptide” antibiotics are assembled by large,multifunctional enzymes which activate, modify, polymerize and in somecases cyclize the subunit amino acids, forming polypeptide chains. Otheracids, such as aminoadipic acid, diaminobutyric acid, diaminopropionicacid, dihydroxyamino acid, isoserine, dihydroxybenzoic acid,hydroxyisovaleric acid, (4R)-4-[(E)-2-butenyl]-4,N-dimethyl-L-threonine,and ornithine can also be incorporated (Katz et al. (1977)Bacteriological Review 41:449–474; Kleinkauf et al. (1987) Annual Reviewof Microbiology 41:259–289). The products are not encoded by any mRNA,and ribosomes do not directly participate in their synthesis. Peptideantibiotics synthesized non-ribosomally can in turn be grouped accordingto their general structures into linear, cyclic, lactone, branchedcyclopeptide, and depsipeptide categories (Kleinkauf et al. (1990)European Journal of Biochemistry 192:1–15). These different groups ofantibiotics are produced by the action of modifying and cyclizingenzymes; the basic scheme of polymerization is common to them all.Non-ribosomally synthesized peptide antibiotics are produced by bothbacteria and fungi, and include edeine, linear gramicidin, tyrocidineand gramicidin S from Bacillus brevis, mycobacillin from Bacillussubtilis, polymyxin from Bacillus polymiyxa, etamycin from Streptomycesgriseus, echinomycin from Streptomyces echinatus, actinomycin fromStreptomyces clavuligerus, enterochelin from Escherichia coil,gamma-(α-L-aminoadipyl)-L-cysteinyl-D-valine (ACV) from Aspergillusnidulans, alamethicine from Trichoderma viride, destruxin fromMetarhizium anisolpliae, enniatin from Fusarium oxysporum, andbeauvericin from Beauveria bassiana. Extensive functional and structuralsimilarity exists between the prokaryotic and eukaryotic systems,suggesting a common origin for both. The activities of peptideantibiotics are similarly broad, toxic effects of different peptideantibiotics in animals, plants, bacteria, and fungi are known (Hansen(1993) Annual Review of Microbiology 47:535–564; Katz et al. supra;Kleinkauf et al. supra; Kolter et al. (1992) Annual Review ofMicrobiology 46:141–163).

The “aminoglycosides” and other “carbohydrate-containing” antibioticsrefer to organic molecules derived at least part from a saccharide orpolysaccharide. For instance, the aminoglycosides are oligosaccharidesconsisting of an aminocyclohexanol moiety glycosidically linked to otheramino sugars. Streptomycin, one of the best studied of the group, isproduced by Streptomyces griseus. Streptomycin, and many otheraminoglycosides, inhibits protein synthesis in the target organisms.

The “ribosomally-synthesized peptide” antibiotics are characterized bythe existence of a structural gene for the antibiotic itself, whichencodes a precursor that is modified by specific enzymes to create themature molecule.

The term “variegated population” refers to a population of, e.g., cells,vectors, or the like, including multiple different species. A variegatedpopulation of cells preferably includes at least 10², 10³, 10⁴ or 10⁵different phenotypes in the cell population. Likewise, a variegatedpopulation of vectors preferably includes at least 10², 10³, 10⁴ or 10⁵different vectors.

III. Sources for Microbial Cellular DNA

As set out above, the methods of the present invention will allow accessto the microbial genetic information present in an environment,particularly those having complex microbial communities, withoutrequiring knowledge of any particular organism or the ability to cultureit. The microorganisms from which recombinant genomic libraries may beprepared include prokaryotic microorganisms, such as eubacteria andarchaea, and lower eukaryotic microorganisms such as fungi, some algaeand protozoa. The subject methods are based, in part, on theunderstanding that noncultured microbes can constitute the vast majorityof the total microbes in any environment, including heavily sampledenvironments such as soil (for a review, see Amann et al. (1995)Microbiol. Rev. 59:143–169).

In preferred embodiments, the libraries of genomic DNA ultimatelysampled in the present method are can be produced by directlysub-cloning genomic DNA from a complex microbial sample without anintervening step of culturing cells from the sample. In techniquesrequiring an intermediate culturing steps, populations of micoroorganismcan be lost due the inability of any single culture conditions touniformity propagate cells in a complex mixture of microorganisms. TheDNA recovered in the subject method is understood to be relativelyunbaised in this respect. Sources, for example, of microbes from whichthe genomic library clones are obtained include, but are not limited to,such environmental samples as may be isolated from soil, insectintestines, plant rhizospheres, microbial mats, sulfur springs, oceanand fresh water ecosystems, etc. In certain embodiments, the genomic DNAcan be obtained from extreme environments, such as from samples ofarctic or antarctic ice, water or permafrost sources, samples fromenvironments of extreme pH (acidic or basic), samples from volcanicorigins or other high temperature and/or high sulfur environments,samples from soil or plant sources of tropical origin, and the like.Each of the above sample sources are representative of meaningfulenvironments that can be exploited by the subject method as each likelycontains a great population of unculturable microorganisms orunculturable combinations of microorganisms. Moreover, many of theseenvironments have not been heavily examined for natural products, etc.The microbial source from which genomic DNA is to be isolated preferablyincludes at least 100 different microorganisms, more preferably at least10³, and even more preferably at least 10⁴, 10⁵ and even 10⁶ differentmicroorganisms.

To further illustrate, many invertebrates have been shown to have adiverse collection of microbes associated with their digestive systems.See, for example, Amann et al. (1995) Microbiol. Rev. 59:143–169. Themicrobes in these environments are phylogenetically diverse andphysically accessible. For example, termite intestines containrepresentatives of the proteobacteria, spirochetes, bacteroides and lowG+C Gram-positive groups, as well as members that may represent novelbacterial and archaeal phyla altogether (Ohkuma et al. (1996) Appl.Environ. Microbiol. 62:461–468). The population of gut microbes is oftenrich in microbes which are unculturable by existing methods; thereforean approach that does not rely on culturing should be successful ingaining access to the genetic information of these microbes. Advantagesof obtaining DNA samples from this environment include the fact that thegenes of a large proportion of these microbes may be easy to express inE. coli and the like, since many gut microbes are proteobacteria, thesame metabolically diverse phylum to which E. coli belongs.

In another embodiment, genomic DNA can be isolated from microorganismspresent in soil (land or marine). Soil microbes have been anunparalleled source for natural product discovery based on conventionalapproaches. Moreover, Applicants' work has revealed that a wide range ofpreviously unaccessed microbes exist in soil samples (Bintrim et al.(1997) PNAS 94:277–282; Bintrim et al., in press). It is expected, forexample, that genomic DNA from a range of different bacteria, archaeaand other microbes can be isolated from various soil samples, and themajority of that DNA is expected to be from previously unculturedmicrobes.

At the microscopic scale, soil is extremely heterogeneous and consistsof numerous microenvironments that differ in many chemical and physicalproperties. To access the microbial diversity of soils, microbiologistshave long relied on standard microbial cultivation techniques. However,the microbes that were cultivated from soil, as a whole, indicatedneither the abundance nor the phylogenetic diversity in situ. It isestimated that fewer than 1% of the microbes observed by microscopy aregenerally recovered by cultivation under standard conditions. Applicantsunderstand that the difference between microflora counted by cultivationand that observed by direct microscopy is largely due to the presence insoil of a vast and as yet uncharacterized taxonomic diversity which arenot readily accessible by presently available culturing techniques. Theinstant methods, by utilizing direct DNA isolation techniquesindependent of microbial cultivation, can be well suited for the generalcloning of genomic DNA from soil microflora.

A number of approaches can be taken to prepare DNA from soil microbesand the like, including: direct isolation, and separation of microbialcells from the environmental support followed by cell lysis and DNApurification. The first method maximizes the amount and diversity of DNArecovered, the second method will maximize the size of recovered DNA.Preferably the isolation and lysis methods, e.g., as described below,will result in lysis of a wide variety of microbial cell types so as tominimize species loss at this step. However, depending on the protocol,it will be understood that selectivity can be introduced at such steps.For example, the method described in the appended examples for theisolation of genomic DNA from Bacillus cereus is not expected to yieldsignificant fungal DNA contamination, since fungal cells will not belysed by this method. Depending on the host cell, minimization ofeukaryotic DNA in the samples can increase the “productivity” of thelibrary, e.g., since bacterial genes are more likely to be expressed inthe prokaryotic host cell.

To explain by example, in one embodiment of a direct isolation protocol,a soil sample is mixed with extraction buffer and treated with proteaseand SDS. The mixture is centrifuged and the supernatant is extracted,e.g., with chloroform. The DNA is precipitated, such as withisopropanol, and purified on a low melting-point agarose gel or thelike.

However, in preferred embodiments the subject method relies on theisolation of microbes from the source sample, such as soil, followed byextraction of genomic DNA from the isolated microbes. In the presentinvention, microbial isolation followed by DNA purification, rather thandirect extraction of DNA from the source sample, e.g., soil, is designedto facilitate the isolation of very high molecular weight DNA from thesample. Microbes can be isolated directly from soil samples usingpreviously developed methods applicable to a variety of soils and whichprovide maximum diversity. See, for example, O'Donnell et al., In C.Edwards (ed.), Monitoring Genetically Manipulated Microorganisms in theEnvironment, John Wiley and Sons, Chichester; Smith et al. (1994) In K.Ritz, J. Dighton and K. E. Giller (eds.), Beyond the Biomass, John Wileyand Sons, Chichester.

In either embodiment, to isolate the genomic DNA, the microbial cellsmust be lysed. To that end, a variety of means are available for lysingrecalcitrant organisms. For example, a common method for the mechanicallysis of fungi requires the sample to be alternately vortexed with glassbeads and cooled in an ice bath. The cellular extract is recovered bycentrifugation after puncturing the bottom of the tube. Similarly, aMini-Beadbeater TM has been used for lysing bacterial and archaealspecies, where cells are ruptured by vigorous shaking with phenol andzirconium beads. See, Hurley, et al. (1987) J Clin Micro, 25:2227–2229.

Methods for lysis of soil bacteria have included multiple cycles offreeze-thawing, and passage through a French press, which is ahigh-pressure shearing device. One recent method for lysing thesebacteria calls for the successive application of sonication, microwaveheating, and thermal shocks. See, Picard, et al. (1992) Applied andEnvironmental Microbiology, 58:2717–2722.

Another common approach for lysis of microorganisms has involved enzymesthat attack the cell walls. For example, lyticase has proven effectivein lysing fungi, while achromopeptidase, mutanolysin, or proteinase Kremoves cell walls from most Gram-positive microorganisms. See, e.g.,Kaneko et al., (1973) Agr. Biol. Chem., 37:2295–2302; Bollet, et al.(1991) Nucleic Acids Research 19:1955; Siegel et al. (1981) Infectionand Immunity 31:808–815.

However, in preferred embodiments, to construct the large insert DNAlibraries, the microbes are first embedded in agarose plugs ormicrobeads. The agarose acts as a solid yet porous matrix which allowsfor the diffusion of various reagents for DNA purification andsubsequent manipulations while preventing the DNA from being sheared. Insome instances, microbeads are preferred over plugs because the use ofbeads increases the surface area surrounding the sample by approximately1000 fold thereby allowing for more efficient and rapid diffusion ofchemicals and enzymes into and out of the agarose beads. Once embedded,the cells are lysed and proteins degraded in the presence of, e.g., 0.5M EDTA, 1% sarcosyl, and 0.1–1.0 mg/ml of proteinase-K. After cell lysisand protein degradation, the remaining DNA is suitable for enzymaticmodifications.

In an illustrative embodiment, a soil sample can be homogenized orshaken in buffer to disperse soil clumps. The sample is then treatedwith a mild detergent and/or cation-exchange resin to dissociatemicrobial cells from soil particles (O'Donnell et al., supra). Microbesare then separated from the soil by differential centrifugation. Finalpurification of microbes can be by density gradient centrifugation oraqueous two-phase partitioning (O'Donnell et al., supra). As describedin the appended examples, recovery of in excess of 40%–60% of the totalmicrobe diversity of the sample can be achieved using these methods.These microbial preparations will be the source of DNA for constructionof the library.

In another embodiment, flow cytometry techniques can be used to isolatemicroorganisms from biological and non-biological debris with which theymay be associated in an intial sample, e.g., before lysing the cells toisolate genomic DNA. See, for example, Davey et al. (1996) Microbiol Rev60:641; and Porter et al. (1997) Soil Biol Biochem 29:91. Flow cytometryhas provided means for the rapid detection, identification, andseparation of cells, including microbes. The cells can be identified,e.g., by fluorescence activated cell sorting (FACS) techniques, bydetecting an endogenous autofluorescence which many cells possess (e.g.,because of phycobiliproteins or other pigments), or by detecting thepresence of a FACS-detectable vital stain. Vital stains are, e.g., thosewhich penetrate living and dead cells at different rates. For example,brilliant cresyl blue or trypan blue may be used. Which ever method isused to isolate cells by flow cytometry will preferably be carried outin a manner which is indiscriminate for the type of cell. For example,Gram-negative bacteria absorb positively charged stains andGram-positive bacteria absorb negatively charged stains. Thus, either asingle stain which stains both cell-types, or a cocktail of stains whichstains both cell-types should be used.

Flow cytometry methods and equipment are well known in the art andreadily adpated for use in the subject assay. In recent years,optical/electronic instrumentation for detecting fluorescent labels onor in cells has become more sophisticated. For example, flow cytometrycan be used to sort cells at a rate exceeding 25,000 cells per second.These instruments can excite fluorescence at many wavelengths of the UV,visible, and near IR regions of the spectrum

In some instances, unusual amounts of endogenous nucleases can aggravatethe problem of recovering intact nucleic acids. For example, one of thefew groups that has successfully extracted intact DNA from Trichomonasvaginalis reports that this organism is characterized by a high level ofendogenous nuclease activity, and that its DNA is unusually susceptibleto degradation during isolation. See, Riley, et al. (1992) J. Clin.Microbiol. 30:465–472. However, broad spectrum protease and nucleaseinhibitors can be used to inhibit the activity of these enzymes withregard to fragmentation of genomic DNA samples.

In general, the isolation of genomic DNA from a source of microorganismscan be carried out with any appropriate technique which yields highmolecular weight DNA, e.g., with an average length of at least 25 kb,more preferably with an average length of at least 50 kb, and even morepreferably with an average length of at least 75 or 100 kb. Procedureswhich may be used include agarose gel electrophoresis, pulsed field gelelectrophoresis, density gradient centrifugation and fluoresenceactivated sorting. In addition to providing suitably sized fragments ofgenomic DNA, the fractionation step can also be designed to facilitateseparation of the DNA from potential inhibitors of enzymatic reactions.

To illustrate, in one embodiment genomic DNA isolated from the samplecells is size fractionated using a pulsed field gel electrophoresisprotocol. Pulsed field gel electrophoresis (PFGE) is capable ofresolving a wide size range of DNA molecules which would all co-migratein conventional agarose gels. The art, for example, describes pulsedfield gel conditions which permit DNA fragments of up to 250 kilobases(kb) to be separated. Birren et al. (1993) in Pulsed Field GelElectrophoresis. Academic Press, San Diego; and Birren et al. (1994)Nucleic Acids Res 22:5366–70. The separations, which can employcommercially available gel boxes, can be achieved using conditions whichhave been described for traditional pulsed field conditions. With DNAfragments of several hundred kb and smaller, higher field strengths maybe used, resulting in still greater increases in separation speed.

In another illustrative embodiment, DNA samples can be enriched for highmolecular weight fragments by flow cytometry-based separationtechniques. Several flow cytometry protocols are known in the art, andprovide ultrasensitive fluorescence detection technique which can beadapted to the subject method for sizing large DNA fragments, e.g., upto about 175 kb in length. In one technique, fluorescence bursts arerecorded as individual, dye-stained DNA fragments pass through a lowpower, focused, continuous laser beam. The magnitudes of thefluorescence bursts are linearly proportional to the lengths of the DNAfragments. This method has been demonstrated to be well suited tocharacterizing PAC/BAC clones and can be adapted for use in theenrichment of large inserts for the subject libraries. Huang et al.(1996) Nucleic Acids Res 24: 4202–9.

To further illustrate, genomic DNA can be isolated from cellsimmobilized in agarose plugs. The DNA can be partially digested in situand run into a pulsed field gel for isolation, e.g., using aGeneNavigator System (Pharmacia Biotech). The DNA fragments can besize-selected to control the average insert size, and in certainembodiments will preferably be selected for sub-cloning from samples DNAfrom 100 to 500 kb in length. Protocols for enzymatic manipulation ofDNA have been developed for digestion, modification, and ligation of DNAin gel slices (Birren et al. (1994) Nucleic Acids Res 22:5366–70). Theenriched DNA can then be cloned into a suitable vector, such as the BACvector pBeloBAC11, and introduced into the host strain, e.g., byelectroporation (Kim et al. (1996) Genomics 34:213–218; and Shizuya etal. (1992) PNAS 89:8794–8797). The average insert size of the clones canbe determined by the analysis of multiple clones. In many embodiments,it will be useful to use multiple different restriction enzymes for thecloning procedure, and in particular to use two enzymes, such as HindIIIand BamHI, which have recognition sequences that differ in GC content.The GC content of the DNA in the sample is expected to vary, anddigestion conditions will be chosen to maximize representation in thelibrary of DNAs with different GC content.

With further regard to assembling genomic DNA contructs, where genomicfragments of sub-optimal length are initially isolated, the method of“chromosomal building” can be used to create longer fragments. Thismethod allows rapid construction of large pieces of defined DNA in Ffactor-based vectors. The method relies on a combination of general andsite-specific recombination to join large pieces of DNA from smaller,overlapping cloned segments in vivo.

Additionally, the practitioner can get an estimate of the phylogeneticdiversity represented by the DNA cloned in the library by screening forrRNA sequences from, e.g., specific phyla of Bacteria or Archaea. Clonescan be pooled into groups and DNA prepared from the pools. Then PCRamplification of rRNA sequences will be accomplished with primersspecifically hybridizing to rRNA sequences of a given phylogeneticgroup, but not to E. coli rRNA sequences (Amann et al. (1995) Microbiol.Rev. 59:143–169; Manz et al. (1996) Microbiology 142:1097–1106). Thepresence of rRNA genes from organisms of different phyla will indicatethat the library contains DNA from diverse sources. This kind ofanalysis will be useful to determine, for example, that DNA from adiverse range of microorganisms was cloned.

IV. Expression Vectors

The library of genomic clones can be prepared, as described above,without the need for culture expansion, amplification or othersupplementary procedures. The resulting genomic DNA sequences areligated into vectors suitable for maintenance of large DNA sequences inthe desired host cell. In addition to stable maintenance of the largegenomic fragments, the choice of vector is also greatly influenced bythe requirement that all, or substantially all, of the protein codingsequences (open reading frames) present in the genomic fragment betranscribable from the vector in the host cell. To this end, the vectormay include transcriptional regulatory sequences operably linked to thegenomic insert so as to promote or enhance expression of at least aportion of the heterologous coding sequences. However, it is more likelythat expression of the recombinant genes will rely on transcriptionalactivation by the endogenous regulatory sequence of the genomic insert.In either circumstance, the tertiary structure of the resulting vectorshould provide access for transcriptional factors and polymerasecomplexes, in the host cell, to at least a substantial portion of thegenomic insert. Moreover, the vector will preferably include at leastone origin of replication which is functional in the host cell, as wellas one or more selectable marker genes for maintenance of the vector.

Representative examples of vectors which may be used include viralvectors, phage, plasmids, phagemids, cosmids, phosmids, bacterialartificial chromosomes (BACs), bacteriophage P1, P1-based artificialchromosomes (PACs), yeast artificial chromosomes (YACs), yeast plasmids,and any other vectors suitable for a specific host cell and capable ofstably maintaining and expressing a genomic DNA insert of at least 20kb, and more preferably greater than 50–75 kb.

Standard recombinant DNA techniques involve the in vitro construction ofplasmid and viral chromosomes that can be transformed into host cellsand clonally propagated. These cloning systems, whose capacities forexogenous DNA range up to 50 kilobase pairs (kb), are well suited to theanalysis and manipulation of small gene clusters from organisms in whichthe genetic information is tightly packed, as is the case with manymicrobes. It is increasingly apparent, however, that many of thefunctional genetic units of interest may span enormous tracts of DNA.

Preferred vectors for the present invention are the so-called artificialchromosomes. One feature of these vectors is their ability to carrylarge genetic inserts, e.g., greater than 50 kb, with enough mitotic andmeiotic stabilities to make their genetic manipulation straightforward.P1 and PAC/BAC clones can contain high molecular weight inserts (75–100kb or 120+ kb); about four to six times larger than Lambda, and two tothree times larger than Cosmids. In addition, the low copy number of theP1, PAC or BAC vector, e.g., in a restriction andrecombination-deficient E. coli host, confer vastly improved stabilityon these clones. The upper limit on the size of the insert is oftengreat enough that thousands of genes can be included on one vector.Thus, a single vector could provide, through inclusion of gene clusters,all the genes to a specific biosynthetic pathway.

P1-based artificial chromosomes (PACs) and bacterial artificialchromosomes (BACs) have significantly expanded the size of fragmentsfrom eukaryotic genomes that can be stably cloned in E. coli and thelike as plasmid molecules. Advantages of these system include the lowcopy number of the vector (based on the single copy F plasmid of E.coli), large possible insert size (clones containing inserts of up to300 Kb have been propagated), stability of clones in vivo, high cloningefficiency, and easy manipulation of clones by standard techniques(Shizuya et al. (1992) PNAS 89:8794–8797). See also FIG. 2. The BAC andPAC systems provide a method to construct a stable library of largeinserts, which in certain instances can be critical to the success ofthe subject method. See FIGS. 3 and 4. Large inserts may needed, forexample, because a biosynthetic gene cluster(s) of interest may belarge, and because large insert size will maximize the total geneticmaterial represented in the library. Biosynthetic genes for secondarymetabolites, for example, are in most cases clustered in one region ofthe chromosome, along with the genes for self-resistance andpathway-specific regulatory genes. Thus, it is probable that entirepathways can be cloned in one large DNA fragment (Vining et al. (1995)Genetics and Biochemistry of Antibiotic Production,Butterworth-Heinemann, Boston), including the genes required to conferresistance on the host. Additionally, secondary metabolites are usuallymade from simple primary metabolites, such as amino acids, acetate, orcommon sugars. Many of these building blocks are likely to be present inthe E. coli cell. Expression of even a tiny fraction of cloned geneswill mean success for this project in terms of the discovery of novelcompounds.

The utility of the BAC and PAC systems in large-scale genomic mappingefforts has led to the development of protocols optimized specificallyfor these plasmids with large inserts (Birren et al. (1993) in PulsedField Gel Electrophoresis. Academic Press, San Diego; Sheng et al.(1995) Nucl. Acids Res. 23:1990–1996; and Wang et al. (1995)Electrophoresis 16:1–7), and be readily adapted to construction of BACand PAC libraries of microbial DNA. Moreover, genes from diverseprokaryotes such as Thermotoga, Synechocystis, Chromatium, Clostridium,Lactobacillus, Corynebacterium, Bacteroides, and Leptospira can beexpressed in E. coli either from their own promoters or frompromoter-like sequences present within the cloned DNA. See, for example,Black et al. (1995) J. Bacteriol. 177:1952; Buysens et al. (1996) Appl.Environ. Microbiol. 62:865; Chavez et al. (1995) Plant Mol. Biol.28:173; DeLong et al. (1992) PNAS 89:5685; and Ding et al. (1993) J.Gen. Microbiol. 139:1093; Ferreyra et al. (1993) J. Bacteriol. 175:1514.These species represent seven different phyla of bacteria, anddemonstrate that a very wide diversity of heterologous gene expressionsignals can be utilized in such host cells as E. coli. Highly efficientgene expression (including transcriptional, translational, andpost-translational processes) will obviously not occur in all cases.There will be unavoidable selections and limitations introduced in themanipulation and expression of genetic material isolated directly fromthe environment. However, purely on stochastic grounds, the vastmicrobial diversity in the sampled environment means that many geneswill be successfully expressed.

In preferred embodiments, the subject method utilizes cloning vectorsthat are based on the E. coli F-factor replicon. This features allowsfor strict copy number control of the clones so that they are stablymaintained at 1–2 copies per cell. The stability of the cloned DNAduring propagation in an E. coli host cell is substantially higher inlower copy number vectors than in multi-copy counterparts (Kim et al,NAR 20:1083–1085). The stabilizing effect of BAC and Fosmid vectors isnotable especially for certain genomic DNA that are normally unstable inhigh copy number vectors. This includes genomes of Archaeal origins.

As an exemplary embodiment, the present method utilizes the pBeloBAC11vector. See, FIG. 1, and, for example, Zimmer et al. (1997) Genomics42:217–226; and Cai et al. (1995) Genomics 29:413–425. The pBeloBAC11vector represents the second generation BAC cloning vectors, which wasdeveloped from the pBAC108L by introducing the LacZ gene to facilitaterecombinant identification with blue and colorless (white) phenotypes.pBeloBAC11 is a mini-F factor based plasmid. There are three uniquecloning sites: Bam HI, SphI, and Hind III, which are flanked by the T7and SP6 promoters. These promoters can facilitate generating RNA probesfor chromosome walking and DNA sequencing of the insert fragment at thevector-insert junction. The G+C rich restriction sites (Not I, Eag I,Xma I, Sma I, Bgl I, and Sfi I) can be used to excise the inserts of BACclones. There are two selective markers for cloning purposes: LacZ genefor recombinant selection and CMR (chloramphenicol) for transformantselection. The F factor codes for genes that are essential to regulateits own replication and controls its copy number in a cell. Theregulatory genes include oriS, repe, parA, and parB. The oriS and repemediate the unidirectional replication of the F factor, and the parA andparB maintain copy number at a level of one or two per cell. BAClibraries are generated by ligating size-selected restriction digestedDNA with pBeloBAC11 followed by, for example, electroporation into E.coli. This vector allows lacZ-based positive color selection of the BACclones that have insert DNA in the cloning sites at the time of libraryconstruction.

The construction of BAC libraries using pBeloBAC11 can be carried out byany of number of ways. Merely for illustration, the vector is firstdigested with HindIII, Bam H1 or SphI and then dephosphorylated toprevent self ligation. Next, high molecular weight DNA is partiallydigested with HindIII, Bam H1 or SphI, or linkers containing such sitesare added as flanking sequences thereto, and size-selected DNA areligated into the vector. The vector can then electroporated intoappropriate host cells. Recombinant transformants are selected on mediacontaining chloramphenicol, X-Gal, and IPTG. After recombinanttransformants are detected, their size can be assayed by a simpleplasmid DNA minipreparation followed by digestion with NotI to free theDNA insert from the vector, and CHEF electrophoresis. The most widelyused E. coli strain for BAC cloning is DH10B (Hanahan, (1983) J. Mol.Biol. 166:557–580). Key features of this strain include mutations thatblock: 1) restriction of foreign DNA by endogenous restrictionendonucleases (hsdRMS); 2) restriction of DNA containing methylated DNA(5′ methyl cytosine or methyl adenine residues,and 5′ hydroxymethylcytosine) (mcrA, mcrB, mcrC, and mrr); 3) recombination (recA1).

Another family of vectors which can be used in the subject method arethe PAC vectors. The PAC vectors have most of the features of the BACsystem however the vectors contains the SacB gene which provides apositive selection for recombinant clones during library construction.SacB encodes sucrose synthase. When cells are grown in the presence ofsaccharose, sucrose synthase will degrade saccharose into levan which ishighly toxic to E. coli. The BamHI cloning site is within the SacB geneand thus disruption of the SacB gene by insertion of a large DNAfragment allows for growth of the cell on media containing saccharose.Additionally the vector has a “pUC19-link”, containing a high copynumber origin of DNA replication, which is used for convenient vectorpropagation and is later removed during vector preparation for libraryconstruction.

Still another suitable BAC vector is the pFOS1 vector, which is a singlecopy cosmid vector constructed by fusing pBAC108L and pUCcos (a pUCvector in which the region including lacZ and multiple cloning sites wasreplaced by lambda cos sequence). In vivo homologous recombinationbetween two vectors via cos sites resulted in pFOS1. The vector isextremely unstable in most of E. coli strains due to the presence ofdouble cos sites. pop2136 strain (Methods in Enzymology vol. 152pp173–180, 1987), for no apparent reason, can maintain pFOS1 (and otherdouble-cos cosmid vectors) with some stability. The bireplicon is drivenby the pUC replication origin, and exists in high copies in E. coli.After in vitro packaging and transfection to E. coli, the structure ofFosmids is exactly the same as pBAC108L clones except the size;therefore Fosmids are mini-BACs with 40 kb inserts. Fosmid library caneasily be constructed using the protocol for constructing cosmidlibraries with double-cos vectors. The Fosmid system is useful forquickly generating miniBAC libraries from small amounts of source DNA,such as flow-sorted chromosomal DNA.

The subject vectors will generally contain a selectable marker gene.This gene encodes a protein necessary for the survival or growth oftransformed host cells grown in a selective culture medium. Host cellsnot transformed with the vector containing the selection gene will notsurvive in the culture medium. Typical selection genes encode proteinsthat (a) confer resistance to antibiotics or other toxins, e.g.,ampicillin, neomycin, methotrexate, or tetracycline, (b) complementauxotrophic deficiencies, or (c) supply critical nutrients not availablefrom complex media, e.g., the gene encoding D-alanine racemase forBacilli. As set out above, the pBeloBAC11 vector includes a geneproviding chloramphenicol resistance for transformant selection.

In certain instances, it may desirable to express the genomic orfs in aeukaryotic cell, such as a fungal host cell. Functional characterizationof genes within a given PAC or BAC clone can be carried out bytransferring the DNA into eukaryotic cells for transient or long-termexpression. To facilitate transfection studies, the vector can beengineered to include a marker gene which is selectable in theeukaryotic host cell. These retrofitting protocols may be applied with anumber of markers of interest to extend the functionality of PAC and BAClibraries, and specialized aspects of such manipulation of E. coli-basedartificial chromosomes are outlined in, for example, Mejia et al. (1997)Genome Res 7:179–86.

The vector should, as pointed out above, include at least one origin ofreplication for the host cell into which the vector is to betransfected. If also necessary, the vector can include one or morecopy-control sequence for controlling the number of copies of the vectorin any one cell. By way of illustration, for use in E. Coli and otherbacterial host cells, the vector preferably includes one or morebacterial origins of replication (Ori), and preferably ones which do notadversely affect gene expression in infected cells. For example, thebacterial Ori can be a pUC bacterial Ori relative (e.g., pUC, colEI,pSC101, p15A and the like). The bacterial origin of replication canalso, for example, be a RK2 OriV or f1 phage Ori. The vectors alsofurther inlcude a single stranded replication origin, such as an f1single-stranded replication origin.

The vector is transfected into and propagated in the appropriate host.Methods for transfecting the host cells with the genomic DNA vector canbe readily adapted from those procedures which are known in the art. Forexample, the genomic DNA vector can be introduced into the host cell bysuch techniques as the use electroporation, precipitation withDEAE-Dextran or calcium phosphate, or lipofection.

To further illustrate the use of BAC vectors in the subject method, thefollowing exemplary protocols can be followed or readily adapted for usewith most any BAC vector system of the subject method.

A). Preparation of BAC Vector DNA

Because BAC vectors are single copy plasmids, it can be difficult incertain instances to obtain large amounts of BAC vector DNA. Extra caremay also be needed to minimize the contamination of DNA from the hostcell that consists more than 99% of the total DNA. However, followingsuch procedures as provided below, it is possible to obtain sufficientquantitities of BAC vector DNA (e.g., a few micrograms of pBeloBAC11)from liter cultures. The exemplary procedures are described for E. Colihost cells, though the protocol can be readily adapted for a variety ofother host cells.

-   -   1) Starting from a single colony, grow E. coli strain containing        pBeloBAC11 vector in 3 liters of LB+chloramphenicol (15 ug/ul)        with good aeration overnight. Make sure to take a blue colony on        an X-gal/IPTG plate.    -   2) Harvest the cells by centrifugation, and resuspend the cell        pellet in Solution I (without lysozyme). Use 25 ml Solution I        per liter culture.    -   3) Add lysozyme to 2.5 mg/ml, and mix by inversion.    -   4) Add Solution II (50 ml per liter culture) and mix well by        inversion. Leave on ice for 10 minutes.    -   5) Add 37 ml of Solution III per liter culture. Mix gently by        swirling. Keep on ice for 10 minutes.    -   6) Centrifuge 30 minutes at 8,000 g or higher at 4° C.    -   7) Decant the supernatant and filter it through several layers        of cheesecloth. Add the RNase to a final concentration of 0.1        mg/ml, and incubate at room temperature for 15–30 minutes.    -   8) Using 4 Qiagen-tip 500, pre-purify the supernatant as        instructed by the Qiagen procedure. Qiagen tips are        pre-equilibrated with QBT, then the supernatant is applied, then        washed with large volumes of QC, and eluted by 15 ml of QF per        column.    -   9) Precipitate the DNA by adding 0.7 volume of isopropanol, mix,        and centrifuge 15,000 xg for 30 minutes at 4° C.    -   10) Wash the DNA pellet with ice cold 70% ethanol, and air dry.    -   11) Resuspend DNA in 18.6 ml of TE Add 20.5 g CsCl and dissolve.        This is to be spun in two tubes in Beckman 70.1 Ti rotor.    -   12) Add 0.4 ml of EtBr (10 mg/ml), mix and perform        ultracentrifugation for 2–3 days at 45,000 rpm in a Beckman 70.1        Ti rotor.    -   13) Two bands should be visible under U.V. Isolate the lower        band, extract with isoamylalcohol 3–4 times, and dialyze for a        few hours in TE at 4° C.    -   14) Ethanol precipitate DNA, rinse the pellet with 70% ethanol,        and dissolve DNA pellet in TE, and store at −20° C. Solution I:        25 mM TrisHCl, pH 8.0; 50 mM Glucose Solution II: 0.2 N NaOH; 1%        SDS Solution III: 5 M Potassium Acetate, pH 4.8. Add glacial        acetic acid to a solution of 3 M potassium acetate to achieve a        pH 4.8.        B). Preparation of Source DNA. Ligation and Electroporation        BAC Ligation

DNA should be in low melting agarose, in TAE or stored in 0.05 M EDTA.Dialyze the sample in 50 ml tube at 4° C. against 1×TE, 1×PA for 3–5 hrwith one change of solution. Melt agarose at 65° C. for 10 minutes,transfer tube to 44–45° C. water bath. Add agarase, using about 1.5 Ufor each 100 μl of melted gel. Digest 1 hour at 45° C.

Set up ligation with an approximate molar ratio of vector to insert of10:1. Every time a new batch of DNA is used it is a good idea to set uptrial ligations with varying amounts of vector given the difficulties ofdetermining the concentration of insert DNA with certainty.

A typical reaction would contain 100 ng insert DNA with an average sizeof 200 kb and 36.5 ng vector in a volume of between 120 and 150 μl.

Reaction Mixture: 100 μl DNA, 1.8 μl pBAC (20 ng/ml), 2.0 μl 10×ligation buffer, 2.0 μl 10×PA, 0.5 μl ligase 400 U/μl, 3.7 μl H₂O

Combine insert DNA, vector, PA, and H₂O. Heat 5 minutes at 65° C., coolon ice. Add ligase buffer and enzyme. Mix by slowly stirring contents.Incubate overnight at 16° C.

After ligation, carry out drop-dialysis of sample against approximately25 ml 0.5×TE, 1×PA for 2 hours at room temperature in a 100 mm petridish. 1×PA is a mixture of spermine and spermidine which has a combinedconcentration of 1 mM (Spermidine-4HCl MW 254.6, Spermine-3HCl MW348.6). Dissolve both in water, filter sterilize. Store frozen aliquotsat −20° C. [100× stock=Spermidine 75 mM (0.19 g/10 ml)+Spermine 30 mM(0.104 g/10 ml); 1000× stock=Spermidine 750 mM (1.9 g/10 ml)+Spermine300 mM (1.04 g/10 ml)]

Preparation of Competent Cells and BAC Electroporation

-   -   1) Inoculate flasks of SOB (without Mg++) by diluting a fresh        saturated (overnight) culture of DH10B 1:1000 (i.e., 0.3 ml to a        flask containing 300 ml medium).    -   2) Grow with shaking at 37° C. until OD550 reaches 0.7 (no        higher than 0.8). This should take approximately 5 hr when        shaken at 200 rpm.    -   3) Harvest cells by spinning in GSA rotor for 10 minutes at        5,000 rpm.    -   4) Resuspend pellet in a volume of 10% sterile glycerol equal to        the original culture volume.    -   5) Spin 10 minutes at 5,000 rpm at 4° C.    -   6) Carefully pour off supernatant (pellet will be quite loose)        and resuspend cells again in 10% glycerol equal to the original        culture volume.    -   7) Spin 10 minutes at 5,000 rpm at 4° C.    -   8) Carefully pour off supernatant, resuspend cells in the volume        of glycerol remaining in the centrifuge bottle. Pool the cells        in one small centrifuge tube.    -   9) Spin 10 minutes at 7,000 rpm in SS34 rotor.    -   10) Pour off supernatant and resuspend cells in 10% glycerol,        using a volume of 2.0 ml per liter of initial culture.    -   11) Aliquot to microfuge tubes (100–200 μl per tube) and freeze        quickly in a dry ice-ethanol bath. Store cells at −70° C.        Electroporation    -   1) Wash and UV sterilize cuvettes, place on ice and prepare        culture tubes with 0.5 ml SOC.    -   2) Thaw cells and aliquot 25–30 μl to microfuge tubes on ice.    -   3) Add 1–3 μl of ligation mix, and gently mix by flicking tube        bottom with finger.    -   4) Transfer to cuvette and wipe cuvette dry.    -   5) Electroporate using settings of 100 Ohms, 2.5 kV, and 25 μFa.        This usually gives a time constant of approximately 2.4 msec.    -   6) Immediately rinse contents of cuvette with SOC and transfer        to culture tube using a sterile Pasteur pipet.    -   7) Shake for 45 minutes at 37° C. Spread on LB plates containing        12.5 μg/ml chloramphenicol, 50 μg/ml×Gal and 25 μg/ml IPTG.        C). Purification of BAC DNA Via Mini-Preps

A major advantage of working with BAC clones is the ease with which pureBAC DNA can be isolated via miniprep methods. Alkaline lysis is superiorto boiling methods, producing higher yields with greaterreproducibility, though a significant amount of the DNA may be nicked bythe alkaline treatment and converted from supercoiled to open circularmolecules. While the low copy number of BACs means that relatively muchless DNA is recovered than from multi-copy vectors, sufficient DNA canbe obtained from a few ml of bacterial culture for restriction analysis,hybridization, FISH or PCR. Because the BACs are supercoiled, they areresistant to shear-induced breakage during the isolation, hence evenBACs as large as 350 kb require no extraordinary measures in handlingthe DNA. Although vortexing should be avoided, the miniprepped DNA, itmay be pipetted using regular pipet tips without any detectable damageto the DNA.

Alkaline lysis mini-preps of BAC DNA can be performed by the followingsteps. Unless stated, pauses or incubations are not needed between eachstep. Typical yield of BAC DNA from 3 ml cultures is 100–200 ng.

-   -   1) Inoculate a colony into a 10 ml culture containing 1.5 ml LB+        12.5 μg/ml chloramphenicol.    -   2) Grow overnight at 37° C. by shaking at 200 rpm.    -   3) Transfer the culture to a 1.5 ml microfuge tube.    -   4) Pellet the cells by spinning at full speed in a microfuge for        30 seconds, and aspriate or pour off growth medium.    -   5) Thoroughly resuspend the cell pellet in 100 μl chilled        Solution I using a pipetman.    -   6) Place the tubes on ice and add 200 μl of freshly prepared        Solution II. Cap the tube, mix by inversion 8–10 times and        return tubes to ice. At this stage the cells will lyse and the        solution will grow clear and viscous.    -   7) Add 150 μl of Solution III. Cap tube, mix by inversion 8–10        times and return to ice. The addition of solution III will cause        the formation of a flocculent precipitate.    -   8) Centrifuge for 6 minutes at room temperature at full speed in        a microfuge.    -   9) Transfer the supernatant by pouring to a new microfuge tube.        Any visible debris that is transferred can be removed with a        toothpick or pipet tip.    -   10) Precipitate the DNA by adding 1 ml room temperature 100%        ethanol and mixing by inversion.    -   11) Centrifuge for 6 minutes at room temperature in a microfuge.    -   12) Pour off the supernatant and rinse the pellet by adding 500        μl of room temperature 70% ethanol.    -   13) Pour off the ethanol and drain the tube by resting it        upsidedown on a paper towel. Allow to dry completely.    -   14) Resuspend in 201 μl TE.

-   Solution 1: 25 mM TrisHCl pH 8.0 50 mM Glucose 10 mM EDTA After    cells have been resuspended, add Lysozyme to 2.5 mg/ml

-   Solution 2: 0.2N NaOH 1% SDS

-   Solution 3: 5M Potassium Acetate pH 4.8 This is a tricky solution to    prepare. It is made by adding glacial acetic acid to a solution of    3M potassium acetate to achieve a pH of 4.8. This is accomplished by    adding a minimal amount of water to the potassium acetate and then    adding the acetic acid until the potassium acetate is dissolved and    the pH has reached 4.8.    V. Host Cells

The ideal host strain will be one with the following characteristics:permissive for replication and maintenance of the genomic DNA vector;lack of endogenous natural products that would be active in the screens;high transformation efficiency; ability to express heterologous genesfrom sequences within the insert, and presence of appropriate precursormolecules needed for a biosynthetic pathway created by the expression ofthe recombinant genomic sequences. Given these requirements, a preferredhost cell is E. coli. One particularly useful strain is the E. coliDH10B as a host, since this is the optimal host for cloning large DNAfragments from foreign sources (Sheng et al. (1995) Nuc. Acids Res.23:1990–1996). Since most BAC libraries have been constructed in DH10B,cloning protocols have been optimized for this strain (Sheng et al.,supra; and Birren et al. (1994) Nuc. Acids Res. 22:5366–5370).

Although E. coli can be used as the host, it will be appreciated bythose skilled in the art that a much broader selection of host cellsexist. For instance, once a quantitative assessment of the microbialfauna of an environmental sample has been made, the practioner will beable to identify the more abundant taxa. Based on this information,other expression systems and host cells can be used to build furthergenomic expression libraries. For example, if it were found thatactinomycetes is an abundant group in a noncultured soil community, thenan attempt to maximize the range of expression of genes fromenvironmental DNA could include constructing a BAC system inStreptomyces. These bacteria are amenable to molecular genetictechniques and are a proven source of antibiotics based on culturing,but are substantially different from E. coli in terms of gene expressionand thus, may support production of compounds not possible using E. colias the host. This approach can be complementary to the E. coli system.

Suitable prokaryotes for this purpose include eubacteria, such asGram-negative or Gram-positive organisms, for example,Enterobacteriaceae such as Escherichia, e.g., E. coli, Enterobacter,Erwinia, Klebsiella, Proteus, Salmonella, e.g., Salmonella typhimurium,Serratia, e.g., Serratia marcescens, and Shigella, as well as Bacillisuch as B. subtilis and B. licheniformis (e.g., B. licheniformis 41Pdisclosed in DD 266,710 published Apr. 12, 1989), Pseudomonas such as P.aeruginosa, and Streptomyces. One preferred E. coli cloning host is E.coli 294 (ATCC 31,446), although other strains such as E. coli B, E.coli X1776 (ATCC 31,537), E. coli DH5 alpha , and E. coli W3110 (ATCC27,325) are suitable. These examples are illustrative rather thanlimiting.

In certain embodiments, the host strain can be engineered to lose thefunction of certain genes, and the ability of a genomic clone tocomplement the loss-of-function being assayed for in the detection step.

In still other embodiments, where the source of the genomic DNA is verydiverse from the host cell, the host cell can be engineered to expresstranscription factors, polymerase subunits, etc, cloned from theorganisms representing the source of the genomic DNA. For instance, an Ecoli cell can be engineered to express genes involved in gene expressionin Archaea with the goal of increasing the level of expression of thearchaeal genomic DNA provided in the foreign cell.

VI. Detection Techniques

The ability to detect formation of a new, functional biochemical pathwayin the host cell is important to the practice of the subject methods. Ingeneral, the assays are carried out to detect heterologous biochemicaltransformation reactions of the host cell that produce, for example,(small) organic molecules and the like as part of a de novo synthesispathway, or by chemical modification of molecules ectopically providedin the host cell's environment. The presence of generation of suchmolecules by the host cell can be detected in “test extracts”, e.g.,which may be conditioned media, cell lystates, cell membranes, orsemi-purified or purified fractionation products thereof. The latter maybe, as described above, prepared by classical fractionation/purificationtechniques, including chromatographic separation, or solventfractionation (e.g., methanol ethanol, acetone, ethyl acetate,tetrahydrofuran (THF), acetonitrile, benzene, ether, bicarbonate salts,dichloromethane, chloroform, petroleum ether, hexane, cyclohexane,diethyl ether and the like). Where the assay is set up with a respondercell, e.g., to test the effect of an activity produced by the host cellon a whole cell rather than a cell fragment, the host cell and test cellcan be co-cultured together (optionally separated by a culture insert,e.g., Collaborative Biomedical Products, Catalog #40446).

In certain embodiments, the assay will be set up to directly detect,e.g., by chemical or photometric techniques, a molecular species whichis produced (or destroyed) by a biosynthetic pathway of the recombinanthost cell, e.g., whose production or degradation is dependent at leastin part on expression of the heterologous genomic DNA. In otherembodiments, the detection step of the subject method involvescharacterization of fractionated media/cell lysates (the test extract),or application of the test extract to a biochemical or biologicaldetection system. In other embodiments, the assay indirectly detects theformation of products of a heterologous pathway by observing aphenotypic change in the host cell, e.g., in an autocrine fashion, whichis dependent on the establishment of a heterologous biosynthetic pathwayin the host cell.

In certain embodiments, analogs related to a known class of compoundswill be sought, as for example analogs of alkaloids, aminoglycosides,ansamacrolides, beta-lactams (including penicillins and cephalosporins),carbapenems, terpinoids, prostanoid hormones, sugars, fatty acids,lincosaminides, macrolides, nitrofurans, nucleosides, oligosaccharides,oxazolidinones, peptides and polypeptides, phenazines, polyenes,polyethers, quinolones, tetracyclines, streptogramins, sulfonamides,steroids, terpinoids, vitamins and xanthines. In such embodiments, ifthere is an available assay for directly identifying and/or isolatingthe natural product, and it is expected that the analogs would behavesimilarly under those conditions, the detection step of the subjectmethod can be as straight forward as directly detecting analogs ofinterest in the cell culture media or preparation of the cell. Forinstance, chromatographic or other biochemical separation of a testextract can be carried out, and the presence or absence of an analogdetected, e.g., spectrophotometrically, in the fraction in which theknown compounds would occur under similar conditions. In certainembodiments, such compounds can have a characteristic fluorescence orphosphorescence which can be detected without any need to fractionatethe media and/or recombinant cell.

In related embodiments, whole or fractionated culture media or lysatefrom a recombinant host cell can be assayed by contacting the testsample with a heterologous cell (“test cell”) or components thereof. Forinstance, a test cell, e.g., which can be prokaryotic or eukaryotic, iscontacted with conditioned media (whole or fractionated) from arecombinant host cell, and the ability of the conditioned media toinduce a biological or biochemical response from the target cell isassessed. For instance, the assay can detect a phenotypic change in thetarget cell, as for example a change in: the transcriptional ortranslational rate or splicing pattern of a gene; the stability of aprotein; the phosphorylation, prenylation, methylation, glycosylation orother post translational modification of a protein, nucleic acid orlipid; the production of 2_(nd) messengers, such as cAMP, inositolphosphates and the like. Such effects can be measured directly, e.g., byisolating and studying a particular component of the cell, or indirectlysuch as by reporter gene expression, detection of phenotypic markers,and cytotoxic or cytostatic activity on the test cell.

When screening for bioactivity of test compounds produced by therecombinant host cells, intracellular second messenger generation can bemeasured directly. A variety of intracellular effectors have beenidentified. For instance, for screens intended to isolate compounds, orthe genes which encode the compounds, as being inhibitors orpotentiators of receptor- or ion channel-regulated events, the level ofsecond messanger production can be detected from downstream signallingproteins, such as adenylyl cyclase, phosphodiesterases,phosphoinositidases, phosphoinositol kinases, and phospholipases, as canthe intracellular levels of a variety of ions.

The following examples describe assay formats for natural products whicheffect receptor or ion channel function. However, they also providegeneral guidance for detecting the effects of a test sample on othercellular functions.

Thus, in one embodiment, the GTPase enzymatic activity by G proteins canbe measured in plasma membrane preparations by determining the breakdownof γ³²P GTP using techniques that are known in the art (For example, seeSignal Transduction: A Practical Approach. G. Milligan, Ed. OxfordUniversity Press, Oxford England). When compounds that modulate cAMP aretested, it will be possible to use standard techniques for cAMPdetection, such as competitive assays which quantitate [³H]cAMP in thepresence of unlabelled cAMP.

Certain receptors and ion channels stimulate the activity ofphospholipase C which stimulates the breakdown of phosphatidylinositol4,5, bisphosphate to 1,4,5-IP3 (which mobilizes intracellular Ca++) anddiacylglycerol (DAG) (which activates protein kinase C). Inositol lipidscan be extracted and analyzed using standard lipid extractiontechniques. DAG can also be measured using thin-layer chromatography.Water soluble derivatives of all three inositol lipids (IP1, IP2, IP3)can also be quantitated using radiolabelling techniques or HPLC.

The other product of PIP2 breakdown, DAG can also be produced fromphosphatidyl choline. The breakdown of this phospholipid in response toreceptor-mediated signaling can also be measured using a variety ofradiolabelling techniques.

The activation of phospholipase A2 can easily be quantitated using knowntechniques, including, for example, the generation of arachadonate inthe cell.

In various cells, specific proteases are induced or activated in each ofseveral arms of divergent signaling pathways. These may be independentlymonitored by following their unique activities with substrates specificfor each protease.

In the case of certain receptors and ion channels, it may be desirableto screen for changes in cellular phosphorylation. Such assay formatsmay be useful when the receptor pathway of interest is a receptor kinaseor phosphatase. For example, immunoblotting (Lyons and Nelson (1984)Proc. Natl. Acad. Sci. USA 81:7426–7430) using anti-phosphotyrosine,anti-phosphoserine or anti-phosphothreonine antibodies. In addition,tests for phosphorylation could be also useful when the targetedreceptor itself may not be a kinase, but activates protein kinases orphosphatase that function downstream in the signal transduction pathway.

One such cascade is the MAP kinase pathway that appears to mediate bothmitogenic, differentiation and stress responses in different cell types.Stimulation of growth factor receptors results in Ras activationfollowed by the sequential activation of c-Raf, MEK, and p44 and p42 MAPkinases (ERK1 and ERK2). Activated MAP kinase then phosphorylates manykey regulatory proteins, including p90RSK and Elk-1 that arephosphorylated when MAP kinase translocates to the nucleus. Homologouspathways exist in mammalian and yeast cells. For instance, an essentialpart of the S. cerevisiae pheromone signaling pathway is comprised of aprotein kinase cascade composed of the products of the STE11, STE7, andFUS3/KSS1 senes (the latter pair are distinct and functionallyredundant). Accordingly, phosphorylation and/or activation of members ofthis kinase cascade can be detected and used to quantitate receptorengagement. Phosphotyrosine specific antibodies are available to measureincreases in tyrosine phosphorylation and phospho-specific antibodiesare commercially available (New England Biolabs, Beverly, Mass.).

In yet another embodiment, the targeted signal transduction pathwayupregulates expression or otherwise activates an enzyme which is capableof modifing a substrate which can be added to the cell. The signal canbe detected by using a detectable substrate, in which case lose of thesubstrate signal is monitored, or altenatively, by using a substratewhich produces a detectable product. In preferred embodiments, theconversion of the substrate to product by the activated enzyme producesa detectable change in optical characteristics of the test cell, e.g.,the substrate and/or product is chromogenically or fluorogenicallyactive. In an illustrative embodiment the signal transduction pathwaycauses a change in the activity of a proteolytic enzyme, altering therate at which it cleaves a substrate peptide (or simply activates theenzyme towards the substrate). The peptide includes a fluorogenic donorradical, e.g., a fluorescence emitting radical, and an acceptor radical,e.g., an aromatic radical which absorbs the fluorescence energy of thefluorogenic donor radical when the acceptor radical and the fluorogenicdonor radical are covalently held in close proximity. See, for example,U.S. Pat. Nos. 5,527,681, 5,506,115, 5,429,766, 5,424,186, and5,316,691; and Capobianco et al. (1992) Anal Biochem 204:96–102. Forexample, the substrate peptide has a fluorescence donor group such as1-aminobenzoic acid (anthranilic acid or ABZ) or aminomethylcoumarin(AMC) located at one position on the peptide and a fluorescence quenchergroup, such as lucifer yellow, methyl red ornitrobenzo-2-oxo-1,3-diazole (NBD), at a different position near thedistal end of the peptide. A cleavage site for the activated enzyme willbe diposed between each of the sites for the donor and acceptor groups.The intramolecular resonance energy transfer from the fluorescence donormolecule to the quencher will quench the fluorescence of the donormolecule when the two are sufficiently proximate in space, e.g., whenthe peptide is intact. Upon cleavage of the peptide, however, thequencher is separated from the donor group, leaving behind a fluorescentfragment. Thus, activation of the enzyme results in cleavage of thedetection peptide, and dequenching of the fluorescent group.

In still other embodiments, the detectable signal can be produced by useof enzymes or chromogenic/fluorscent probes whose activities aredependent on the concentration of a second messenger, e.g., such ascalcium, hydrolysis products of inositol phosphate, cAMP, etc. Forexample , the mobilization of intracellular calcium or the influx ofcalcium from outside the cell can be measured using standard techniques.The choice of the appropriate calcium indicator, fluorescent,bioluminescent, metallochromic, or Ca++-sensitive microelectrodesdepends on the cell type and the magnitude and time constant of theevent under study (Borle (1990) Environ Health Perspect 84:45–56). As anexemplary method of Ca++ detection, cells could be loaded with the Ca++sensitive fluorescent dye fura-2 or indo-1, using standard methods, andany change in Ca++ measured using a fluorometer.

As certain embodiments described above suggest, in addition to directlymeasuring second messenger production, the signal transduction activityof a receptor or ion channel pathway can be measured by detection of atranscription product, e.g., by detecting receptor/channel-mediatedtranscriptional activation (or repression) of a gene(s). Detection ofthe transcription product includes detecting the gene transcript,detecting the product directly (e.g., by immunoassay) or detecting anactivity of the protein (e.g., such as an enzymatic activity orchromogenic/fluorogenic activity); each of which is generally referredto herein as a means for detecting expression of the indicator gene. Theindicator gene may be an unmodified endogenous gene of the host cell, amodified endogenous gene, or a part of a completely heterologousconstruct, e.g., as part of a reporter gene construct.

In one embodiment, the indicator gene is an unmodified endogenous gene.For example, the instant method can rely on detecting thetranscriptional level of such endogenous genes as the c-fos gene (e.g.,in mammalian cells) or the Bar1 or Fus1 genes (e.g., in fungal cells) inresponse to such signal transduction pathways as originating from Gprotein coupled receptors.

Many reporter genes and transcriptional regulatory elements are known tothose of skill in the art and others may be identified or synthesized bymethods known to those of skill in the art. Examples of reporter genesinclude, but are not limited to CAT (chloramphenicol acetyl transferase)(Alton and Vapnek (1979), Nature 282: 864–869) luciferase, and otherenzyme detection systems, such as beta-galactosidase; firefly luciferase(deWet et al. (1987), Mol. Cell. Biol. 7:725–737); bacterial luciferase(Engebrecht and Silverman (1984), PNAS 1: 4154–4158; Baldwin et al.(1984), Biochemistry 23: 3663–3667); alkaline phosphatase (Toh et al.(1989) Eur. J. Biochem. 182: 231–238, Hall et al. (1983) J. Mol. Appl.Gen. 2: 101), human placental secreted alkaline phosphatase (Cullen andMalim (1992) Methods in Enzymol. 216:362–368); β-lactamase or GST.

Transcriptional control elements for use in the reporter geneconstructs, or for modifying the genomic locus of an indicator geneinclude, but are not limited to, promoters, enhancers, and repressor andactivator binding sites. Suitable transcriptional regulatory elementsmay be derived from the transcriptional regulatory regions of geneswhose expression is rapidly induced, generally within minutes, ofcontact between the cell surface protein and the effector protein thatmodulates the activity of the cell surface protein. Examples of suchgenes include, but are not limited to, the immediate early genes (see,Sheng et al. (1990) Neuron 4: 477–485), such as c-fos. Immediate earlygenes are genes that are rapidly induced upon binding of a ligand to acell surface protein. The transcriptional control elements that arepreferred for use in the gene constructs include transcriptional controlelements from immediate early genes, elements derived from other genesthat exhibit some or all of the characteristics of the immediate earlygenes, or synthetic elements that are constructed such that genes inoperative linkage therewith exhibit such characteristics. Thecharacteristics of preferred genes from which the transcriptionalcontrol elements are derived include, but are not limited to, low orundetectable expression in quiescent cells, rapid induction at thetranscriptional level within minutes of extracellular simulation,induction that is transient and independent of new protein synthesis,subsequent shut-off of transcription requires new protein synthesis, andmRNAs transcribed from these genes have a short half-life. It is notnecessary for all of these properties to be present.

Other promoters and transcriptional control elements, in addition tothose described above, include the vasoactive intestinal peptide (VIP)gene promoter (cAMP responsive; Fink et al. (1988), Proc. Natl. Acad.Sci. 85:6662–6666); the somatostatin gene promoter (cAMP responsive;Montminy et al. (1986), Proc. Natl. Acad. Sci. 8.3:6682–6686); theproenkephalin promoter (responsive to cAMP, nicotinic agonists, andphorbol esters; Comb et al. (1986), Nature 323:353–356); thephosphoenolpyruvate carboxy-kinase gene promoter (cAMP responsive; Shortet al. (1986), J. Biol. Chem. 261:9721–9726); the NGFI-A gene promoter(responsive to NGF, cAMP, and serum; Changelian et al. (1989). Proc.Natl. Acad. Sci. 86:377–381); and others that may be known to orprepared by those of skill in the art.

In the case of receptors which modulate cyclic AMP, a transcriptionalbased readout can be constructed using the cyclic AMP response elementbinding protein, CREB, which is a transcription factor whose activity isregulated by phosphorylation at a particular serine (S133). When thisserine residue is phosphorylated, CREB binds to a recognition sequenceknown as a CRE (cAMP Responsive Element) found to the 5′ of promotorsknown to be responsive to elevated cAMP levels. Upon binding ofphosphorylated CREB to a CRE, transcription from this promoter isincreased.

Phosphorylation of CREB is seen in response to both increased cAMPlevels and increased intracellular Ca levels. Increased cAMP levelsresult in activation of PKA, which in turn phosphorylates CREB and leadsto binding to CRE and transcriptional activation. Increasedintracellular calcium levels results in activation of calcium/calmodulinresponsive kinase II (CaM kinase II). Phosphorylation of CREB by CaMkinase II is effectively the same as phosphorylation of CREB by PKA, andresults in transcriptional activation of CRE containing promotors.

Therefore, a transcriptionally-based readout can be constructed in cellscontaining a reporter gene whose expression is driven by a basalpromoter containing one or more CRE. Changes in the intracellularconcentration of Ca++ (a result of alterations in the activity of thereceptor upon engagement with a ligand) will result in changes in thelevel of expression of the reporter gene if: a) CREB is alsoco-expressed in the cell, and b) either an endogenous or heterologousCaM kinase phosphorylates CREB in response to increases in calcium or ifan exogenously expressed CaM kinase II is present in the same cell. Inother words, stimulation of PLC activity may result in phosphorylationof CREB and increased transcription from the CRE-construct, whileinhibition of PLC activity may result in decreased transcription fromthe CRE-responsive construct.

Continuing with the illustrative example, the marker gene is coupled tothe receptor signaling pathway so that expression of the marker gene isdependent on activation of the receptor. This coupling may be achievedby operably linking the marker gene to a receptor-responsive promoter.The term “receptor-responsive promoter” indicates a promoter which isregulated by some product of the target receptor's signal transductionpathway.

Alternatively, the promoter may be one which is repressed by thereceptor pathway, thereby preventing expression of a product which isdeleterious to the cell. With a receptor repressed promoter, one screensfor agonists by linking the promoter to a deleterious gene, and forantagonists, by linking it to a beneficial gene. Repression may beachieved by operably linking a receptor-induced promoter to a geneencoding mRNA which is antisense to at least a portion of the mRNAencoded by the marker gene (whether in the coding or flanking regions),so as to inhibit translation of that mRNA. Repression may also beobtained by linking a receptor-induced promoter to a gene encoding a DNAbinding repressor protein, and incorporating a suitable operator siteinto the promoter or other suitable region of the marker gene.

In the case of fungal cells, suitable positively selectable (beneficial)genes include the following: URA3, LYS2, HIS3, LEU2, TRP1; ADE,1, 2, 3,4, 5, 7, 8; ARG1, 3, 4, 5, 6, 8; HIS1, 4, 5; ILV1, 2, 5; THR1 ,4; TRP2,3, 4, 5; LEU1, 4; MET2, 3, 4, 8, 9, 14, 16, 19; URA1, 2, 4, 5, 10; HOM3,6; ASP3; CHO1; ARO 2, 7, CYS3; OLE1; INO1,2,4; PRO1,3 Countless othergenes are potential selective markers. The above are involved inwell-characterized biosynthetic pathways. The imidazoleglycerolphosphate dehydratase (IGP dehydratase) gene (HIS3) is preferred becauseit is both quite sensitive and can be selected over a broad range ofexpression levels. In the simplest case, the cell is auxotrophic forhistidine (requires histidine for growth) in the absence of activation.Activation leads to synthesis of the enzyme and the cell becomesprototrophic for histidine (does not require histidine). Thus theselection is for growth in the absence of histidine. Since only a fewmolecules per cell of IGP dehydratase are required for histidineprototrophy, the assay is very sensitive.

In other embodiments, the reporter gene can be used to detect agentsfrom the test samples which can directly alter the activity of atranscription factor or other DNA associated protein. For instances, thedetection step can be used to identify compounds from the test sampleswhich can inhibit or potentiate transcription of a gene by a cellular orviral transcription factor.

In still other embodiments, the detection step is provided in the formof a cell-free system, e.g., a cell-lysate or purified or semi-purifiedprotein or nucleic acid preparation. The samples obtained from therecombinant host cells can be tested for such activities as inhibitingor potentiating such pairwise complexes (the “target complex”) asinvolving protein-protein interactions, protein-nucleic acidinteractions, protein-ligand interactions, nucleic acid-nucleic acidinteractions, and the like. The assay can detect the gain or loss of thetarget complexes, e.g., by endogenous or heterologous activitiesassociated with one or both molecules of the complex.

Assays which are performed in cell-free systems, such as may be derivedwith purified or semi-purified proteins, are often preferred as“primary” screens in that they can be generated to permit rapiddevelopment and relatively easy detection of an alteration in amolecular target when contacted with a test sample. Moreover, theeffects of cellular toxicity and/or bioavailability of the test samplecan be generally ignored in the in vitro system, the assay instead beingfocused primarily on the effect of the sample on the molecular target asmay be manifest in an alteration of binding affinity with othermolecules or changes in enzymatic properties (if applicable) of themolecular target. Detection and quantification of the pairwise complexesprovides a means for determining the test samples efficacy at inhibiting(or potentiating) formation of complexes. The efficacy of the compoundcan be assessed by generating dose response curves from data obtainedusing various concentrations of the test sample. Moreover, a controlassay can also be performed to provide a baseline for comparison. Forinstance, in the control assay conditioned media from untransformed hostcells can be added.

The amount of target complex may be detected by a variety of techniques.For instance, modulation in the formation of complexes can bequantitated using, for example, detectably labelled proteins or the like(e.g. radiolabelled, fluorescently labelled, or enzymatically labelled),by immunoassay, or by chromatographic detection.

Additionally, the effect of a test sample on a target complex can bedetermined by use of a an interaction trap assay. See, for example, U.S.Pat. No. 5,283,317; PCT publication WO94/10300; Zervos et al. (1993)Cell 72:223–232; Madura et al. (1993) J Biol Chem 268:12046–12054;Bartel et al. (1993) Biotechniques 14:920–924; and Iwabuchi et al.(1993) Oncogene 8:1693–1696). The interaction trap assay relies onreconstituting in vivo a functional transcriptional activator proteinfrom two separate fusion proteins, one of which comprises theDNA-binding domain of a transcriptional activator fused to one of theproteins of the target complex. The second fusion protein comprises atranscriptional activation domain (e.g. able to initiate RNA polymerasetranscription) fused to the other protein of the target complex. Whenthe two protein interact, the two domains of the transcriptionalactivator protein are brought into sufficient proximity as to causetranscription of a reporter gene. Thus, test samples which are able toinhibit or potentiate interaction of the fusion proteins will result inmodulation of the expression of the reporter gene. Versions of theinteraction trap assay also exist for detecting protein-nucleic acid andnucleic acid-nucleic acid interactions and be readily adapted for use inthe subject method.

In still other embodiments, a purified or semi-purified enzyme can beused as to assay the test samples. The ability of a test sample toinhibit or potentiate the activity of the enzyme can be convenientlydetected by following the rate of conversion of a substrate for theenzyme.

In yet other embodiments, the detection step can be designed to detect aphenotypic change in the host cell which is induced by products of theexpression of the heterologous genomic sequences. For instance, theassay can detect the ability of a genomic clone to confer antibioticresistance to the host cell. Many of the above-mentioned cell-basedassay formats can also be used in the host cell, e.g., in anautocrine-like fashion.

In addition to providing a basis for isolating biologically activemolecules produced by the recombinant host cells, the detection step canalso be used to identify genomic clones which include genes encodingbiosynthetic pathways of interest. Moreover, by interative and/orcombinatorial sub-cloning methods relying on such detection steps, theindividual genes which confer the detected pathway can be cloned fromthe larger genomic fragment.

The subject screening methods can be carried in a differential format,e.g., comparing the efficacy of a test sample in a detection assayderived with human components with those derived from, e.g., fungal orbacterial components. Thus, selectivity as an bacterocide or fungicidecan be a criteria in the selection protocol.

The host strain need not produce high levels of the novel compounds forthe method to be successful. Expression of the genes may not be optimal,global regulatory factors may not be present, or metabolite pools maynot support maximum production of the product. The ability to detect themetabolite will often not require maximal levels of production,particularly when the bioassay is sensitive to small amounts of naturalproducts. Thus initial submaximal production of compounds need not be alimitation to the success of the subject method.

Finally, as indicated above, the test sample can be derived from, forexample, conditioned media or cell lystates. With regard to the latter,it is anticipated that in certain instances there may be heterologouslyexpressed compounds which may not be properly exported from the hostcell. For example, violacein is produced by recombinant E. Coli, yet thecolonies turn purple without the plate surrounding the colony turningthat color. This suggests that in order to detect the antibacterialactivity of a violicein-like product, one would need to assay celllysates. Their are a variety of techniques available in the art forlysing cells. A preferred approach is another aspect of the presentinvention, namely, the use of host cell-specific lysis agent. Forinstance phage (i.e. P1, λ, φ80) can be used to selectively lysis Ecoli. Addition of such phage to grown cultures of an E. coli host cellscan maximize access to the heterolgous products of new biosyntheticpathways in the cell. Moreover, such agents would not interfere with thegrowth of a tester organism, e.g., a human cell, which may beco-cultured with the host cell library.

The following examples, though not intending to be limiting in anymanner, provide further guidance.

A) Isolation and Structural Characterization of Active Compounds.

Once clones producing a compound with biological activity have beenidentified, the clones can be grown in large batches and activecompounds purified from the scaled up process. The same cell-based andcell-free assays described above can also be used to monitorpurification. Purification of the activities can be carried out usingany of a number of techniques, and may be based on differentialsolubilities, thin layer chromatography, ion-exchange chromatography,and high-performance liquid chromatography, all of which are commonpractice in the art. Furthermore, structural determinations can be madeon purified and semi-purified preparations of the activity.

B) High-Throughput Robotic Screening of BAC Clones for Production ofNatural Products

The high throughput processing and analysis of large genomic librariesby the subject method can be automated, e.g., using automated/roboticsystems. The automation can include, for instance, such activitiesas: 1) arraying and storage of BAC libraries; 2) growth and separationof cells/conditioned culture media; and 3) testing conditioned media inbiological and biochemical assays. These are outlined below for theexemplary embodiment of a BAC genomic DNA library. The detailedmethodologies will vary from one embodiment to the next, but can bereadily implemented by those skilled in the art.

Arraying and storage of BAC clones: Following ligation of the DNA intoBAC vectors, the ligation mixture is transfected into a suitable hostcell, and BAC-containing colonies are selected. If the number of clonesrecovered is small (e.g., less than 1000), then arraying into glycerolstocks can be accomplished manually. However, if libraries of more than10,000 clones are obtained, then arraying is best accomplished using anautomated colony picking robot.

Growth for expression of natural products and separation of culture:Clones can be inoculated for growth in deep-well 96-well plates, e.g.,by using an automated pipetting station. Growth conditions can beestablished, e.g., using control strains. Following growth, culturemedia is isolated either by centrifugation and removal of supernatantsor by filtration. Residual cells in the isolated culture media can bekilled using chloroform vapors.

High throughput assays: The conditioned media can be tested for activityin high throughput biochemical or biological assays adapted forautomated readouts. For instance, the method can employ establishedprocedures for robotic antimicrobial testing. In general, such assaysare performed in multi-well plates (96 or 384) or by placing smallaliquots of conditioned media onto plates seeded with a bacterial orfungal lawn or the like. The goal is to develop an automated method thatis sensitive and rapid. In addition to antimicrobial assays, asdescribed above the culture supernatants can be tested in biochemicalassays, such as competitive binding assays or enzyme activity assays, aswell as whole cell assays, e.g., which detect changes in phenotypedependent on addition of conditioned media. To increase throughput, itmay be desirable to test pools of culture supernatants in certaininstances.

C) Screen the BAC Libraries for Activity Against Invertebrate Pests andPathogens, Using Nematodes as the Model

In one embodiment, the subject method can be used to create culturemedia which is tested for insecticidal activity. For instance, aliquotsof BAC-expressed culture media can be added to culture of C. elegans,e.g., an easily culturable nematode. In other embodiments, the nematodescan be co-cultured with BAC-transformed bactera (the bacteria providingfood for the nematodes, which are bacterial feeders). If the conditionedmedia or bacteria produce a nematicidal compound, then no growth will beseen. Clones that are active in this assay will be retested in a varietyof insect bioassays to determine insecticidal activity.

VIII. Sequencing

Genomic clones identified in the subject assay can be isolated, and thesequence for the entire genomic fragment, or individual genes thereof,can be obtained by any of a number of sequencing methods known in theart. For instance, Sanger or Maxam and Gilbert sequencing can beperformed. In other embodiments, the sequence can be obtained bytechniques utilizing capillary gel electrophoresis or mass spectroscopy.See, for example, U.S. Pat. No. 5,003,059. Such techniques are preferredfor automation of the sequencing step.

In certain embodiments, it will be desirable to fragment the genomic DNAinsert, e.g., by restriction mapping, sequence the various inserts, andreassemble the full-length sequence. For very large inserts in BAC, PACand P1 clones, it may be difficult to construct detailed restrictionmaps, e.g., with large number of restriction fragments leading toambiguity of mapping data. However, the use of a recombinase tolinearise and asymmetrically introduce a label at the unique recombinaserecognition site of large clones. Subsequent partial digestion allowsthe direct ordering of restriction fragments. Efficient Cre-loxlinearisation of BACs and applications of such techniques to physicalmapping are decribed by, e.g., Mullins et al. (1997) Nucleic Acids Res25:2539–40.

Merely for illustration, the following exemplary description of apreparation protocol will provide guidance for isolation of large clonesfor direct sequencing reactions. This protocol can be used to prepareclones such as BACs, PACs, cosmids or fosmids for direct sequencingreactions. Before starting the growth steps, it is important to find outwhat type of cloning vector was originally used—cosmid, fosmid, PAC orBAC, and what antibiotic resistance marker is carried by the vector.This will determine the culture volume to be prepared, as well as theantibiotic to be used, and its concentration. For instance, cosmids aremulticopy vectors (e.g. many copies of the cosmid exist in each cell),so a 5–10 ml growth volume should be sufficient. On the other hand,fosmids, PACs and BACs are present one per cell (“single copy vectors”),and require a 12–15 ml growth volume to prepare sufficient DNA forsequencing reactions. Thus, the following protocol is based upon a 5 mlgrowth volume, where increased culture volumes will require that reagentvolumes be scaled up accordingly.

-   -   1. Pellet cells from a 5 ml overnight growth by centrifugation        for 5 minutes at 3500 rpm in the Jouan centrifuge, using the        appropriate carrier. Discard the supernatant over a sink and        invert the tubes onto clean paper toweling for 5 minutes to        drain.    -   2. Add 400 μl of GET buffer plus RNase A (use 10 μl of 10 mg/ml        RNase A [DNase free] per 1 ml of GET buffer stock) to each tube.        Mix on the vortexer or by P1000 pipet (up/down pipetting) until        a suspension without clumps of cells is obtained.    -   3. Using a P1000, transfer the resuspended cells to a clean 1.5        ml microcentrifuge tube for each sample.    -   4. Add 400 μl of freshly prepared lysis solution (2 ml of 5 N        NaOH, 5 ml of 10% SDS and 43 ml of ddH2O) to each tube. Mix        gently by inversion and place on ice for 5 minutes.    -   5. Add 400 μl of 3M KOAc, pH 4.8 to each sample. Invert several        times to mix and place on ice for 5 minutes. A thick white        precipitate should form once the solution is mixed.    -   6. Centrifuge for 5 minutes to pellet the chromosomal DNA and        cell debris. While centrifuging, label two clean microcentrifuge        tubes per sample. Place 600 μl of phenol:chloroform per tube.    -   7. Using a P1000, remove 600 μl aliquots of the resulting        supernatant from each prep into the two phenol-containing tubes.        Cap the tubes and vortex for 10–20 seconds. Spin for 5 minutes        to separate phases. While centrifuging, label two clean        microcentrifuge tubes per sample. Place 600 μl isopropanol per        tube.    -   8. Using a P1000, remove the upper phase (˜600 μl) to the        isopropanol-containing tubes. Cap the tubes and invert several        times to mix.    -   9. Centrifuge for 15 minutes to pellet the DNA. Discard the        supernatant.    -   10. Wash each pellet with the addition of 1 ml of 70% ethanol.        Spin 5 minutes and decant the wash. Dry briefly under vacuum.    -   11. Resuspend the DNA pellet in 20 μl* ddH2O. For large clones,        checking the DNA on an agarose gel first requires digestion with        a common restriction enzyme, such as Eco R1 or Hind III. Consult        with library core on their choice for your particular clone, set        up the digest, and electrophorese on a 1% agarose gel for 45–60        minutes to check the digestion. Proceed with sequencing if the        digest appears on the gel as discrete fragments of reasonable        intensity. * Note: this is the only volume in the protocol that        should not be scaled up. In other words, always resuspend the        DNA pellet in 20 μl of water regardless of the starting volume        of culture.

The following protocol can then be used either to sequence directly fromCsCl banded cosmid clones (obtained from Library Core) or from alkalinelysis prepared cosmid, BAC, PAC or fosmid clones according to, forexample, the above procedure for large clone DNA preparation.

Reaction Assembly:

Per sample add the following in a fresh 0.2 ml tube: Taq FS Prism premix8 μl Primer @ 3–10 uM 1 μl cosmid/BAC/PAC or fosmid DNA   μl (400ng/rxn) ddH20 to 20 μl Total 20 μlThermal Cycling: Cycle reactions as follows:

-   -   95° C. for 15 seconds    -   45° C. for 5 seconds    -   60° C. for 2 minutes    -   repeat for a total of 15 cycles    -   4° C. hold        Precipitation: When samples have completed cycling add: 2.0 μl        of 3M NaOAc, pH 5.2 100 μl of 100% EtOH. Then transfer the        samples into a fresh 1.7 ml microcentrifuge tube. Spin reactions        at 13,000 r.p.m. for 15 minutes. Wash 1× with 250 μl of 70% EtOH        with a 5 minute spin. Decant EtOH, dry in speed vac and store at        −2° C. Resuspend in 2 μl of dye and load onto a 377.        BAC End-Sequencing    -   1. For every 4 mls of culture, dissolve the BAC DNA pellet in 40        μl of water. for example: Usually each BAC is grown in 20 mls        LB/CM total, then is dispensed into one Autogen tube (4 mls in        each of the 5 tubes). After miniprep, add 40 μl of water to each        tube (200 μl total for each BAC).    -   2. Vortex the Autogen tube and let sit for at least 0.5 hour.        Then pool the 5 samples into one for each BAC.    -   3. check the BAC DNA for quality and quantity by digesting 5 μl        of the DNA in a 20 μl reaction: 5.0 μl DNA        -   2.0 μl 10× Buffer 2 (NEB)        -   0.5 μl Hind III (NEB)        -   12.5 μl H₂O    -   Digest for 2–4 hours at 37° C.    -   4. Run the digest on a 0.8% agarose gel until the xylene cyanol        line is at least 1 inch below the wells. There should be a        strong band pattern for each BAC.    -   5. If DNA is OK for end-sequencing, then prepare 2 reactions for        each BAC using T7 and SP6 primers (18 mers).    -   1 reaction: 22.0 μM DNA        -   16.0 μM reaction mix (ABI/PE #402122)        -   25 μM T7 or SP6    -   PCR conditions: 96° C. 4 min.        -   then 25 cycles of:            -   96° C. 10 sec.            -   50° C. 5 sec.            -   60° C. 4 min.    -   6. After PCR, purify samples in columns (Pharmacia #27-5340–03).    -   Column protocol:        -   1) vortex column;        -   2) break off tip at bottom;        -   3) place column in eppendorf tube and spin for 1 min. at            3000 rpm;        -   4) add all of reaction (40 μl) to top of gel column and            place in a new tube;        -   5) spin again for 1 min. at 3000 rpm;        -   6) speedvac flow-through until all liquid has evaporated;        -   7) give dried reaction to sequencing facility.            IX. Exemplary Uses

There are a wide range of uses for the natural products which can beidentified by the subject method. Secondary metabolites produced bymicroorganisms, such as fungi, reflect a wide variety of chemicalstructures affecting numerous biological activities in different classesof organisms, including both prokaryotes (bacteria) and eukaryotes(animals, plants, and insects). Antibiotics constitute the largest groupof known bioactive secondary metabolites, acting on such diverseprocesses as cell wall synthesis, DNA replication, and proteinsynthesis. In addition to their use as antibiotics, secondarymetabolites are being successfully developed and used in agriculture aspesticides, herbicides, and anti-parasitic compounds, and in treatingnon-infectious human diseases as inhibitors of enzyme

To further illustrate, in animal therapies, the present method may beused to provide, e.g., angiogenesis inhibitors, insecticidal agents,antibacterial agents, antifungal agents, antiprotazoan agents,antiinflammatory drugs, antiparasitic agents, antitumor agents, cellcycle regulators, cytotoxic drugs, immune stimulants,immunosuppressants, ion channel blockers, fibrinolytic agents, freeradical scavengers, prostaglandins and precursors, vasodilators,hypolipidemic agents, viral inhibitors (including reverse transcriptaseand protease inhibitors), and modulators of microtubule dynamics,receptor-ligand interactions and enzyme activity (inhibitors oractivators). the subject method can also provide biological activitymolecules for use in agricultural applications, such as antibiotics,antifeedants, bactericides, enzymes with antibiosis activities(lysozymes, chitinases, glucanases, cellulases), fungicides, herbicides,pesticides (e.g., antihelminthics, insecticides, acaricides,anticoccidials, antitreponemals, and antitrichomonals), ion channelblockers and promoters, miticides, nematicides, pheromones,siderophores, viricides and the like. The subject method can alsoproduce compounds which have applications in the the food industry, suchas may be useful as enzymes, fatty acids, flavorings, gums, novelcarbohydrates, peptides, pigments and dyes, sweeteners, and vitamins.Still other industrial applications include compounds and/or geneproducts useful in bioremediation (e.g., degradation of pesticides,toxic waste, oil, grease), as biotech enzymes (restriction enzymes, newreporter genes, antibiotic resistance markers), as industrial enzymes(amylases, proteases, lipases, phosphatases), or as new sources ofpolysaccharides (lubricants, thickeners). The ability of the polygenomiclibraries of the subject method, e.g., through combinatorial biologybased on microbial recombination, to create natural products having suchactivities can be assessed using standard methods in the art (see, e.g.,Franco et al. (1991) Crit. Rev. in Biotech. 11:193–276, and referencestherein). The subject method, therefore, can further involve the use of,inter alia, biochemical assays, cell or tissue culture assays, andanimal model systems. Several exemplary embodiments of these assays aredescribed further below.

Antibiotic Activities

In one aspect, the method of the present invention can be used todiscover products in the extracts of the engineered cells which displaysome antibiotic activity, e.g., antibacterial, antifungal and/orantiviral. Historically, discovery of antibiotics occurred throughevaluation of fermentation broths for anti-bacterial or anti-fungalactivity. For instance, many proteobacteria produce β-lactamantibiotics. This has been documented in Chromobacterium, Pseudomonas,Agrobacterium, Serratia, and Erwinia (de Lorenzo et al. (1984) TIBS 9:266). Additionally, production of metabolites having antifungalactivity, such as phenazines and phloroglucinols, have been documentedin Pseudomonas (see, for example, Buysens et al. (1996) Appl. Environ.Microbiol. 62:865–871). Myxobacteria have emerged as major producers ofnovel biologically active compounds (Reichenbach et al., 1993, in ThirdInternational Conference on the Biotechnology of Microbial Products:Novel Pharmacological and Agrobiological Activities. Developments inIndustrial Microbiology Series Volume 33. V. P. Gullo, J. C.Hunter-Cevera, R. Cooper, and R. K. Johnson, eds. Society for IndustrialMicrobiology). Therefore, extracts from the combinatorial gene systemsof the present invention should be an excellent and abundant source ofcompounds (e.g., new metabolites) having antibiotic activities.

Anti-bacterial activities can be identified using a number of standardassays known in the art. For example, a culture of bacteria, such as abacterial lawn, can be contacted with an extract from the host cellculture, e.g., filter paper discs doped with the extract, and the areasof lysis characterized. In other embodiments, the extracts are added toa liquid culture of a target organism, and the inhibition of bacterialcell growth can be determined, e.g., by turbidimetric readings. Inaddition to detecting general effects on bacterial growth and viability,the screening methods of the invention can involve assaying for effectson bacteria-specific structures, enzymes, or processes.

A large number of antifungal compounds have been identified usingclassic approaches, e.g., evaluating samples in primary tests directlyagainst a range of filamentous fungi and yeasts, e.g., Candida albicans,grown in agar plates, or in some cases, directly against phytopathogenicinfestations (Bastide et al. (1986) Mircen J. Appl. Microbiol.Biotechnol. 2:453; and Haruo, (1987) Gendai Kagaku Zokan 9:16). Suchasssays can be readily adapted for use in a detection step of thesubject method. Several examples of fungi-specific targets includechitin and glucan synthases (Selitrennikoff et al., (1983) Antimicrob.Agents Chemother. 23:757; Kirsch et al., (1986) J. Antibiot. 39:1620;and Denisot et al., (1990) 9th Int. Symp. Future Trends in Chemother.,Geneva, March 26 to 28, page 47), and cutinases (Koller et al., (1990)J. Antibiot. 43:734; Umezawa et al. (1980) J. Antibiot. 33:1594).

To further illustrate, compounds which modulate sterol biosynthesis havevaluable pharmacological properties. In particular, they can have apronounced antifungal activity, e.g., such as ketoconazole andterbinafine. These compounds can accordingly be used as medicaments,especially for the control or prevention of topical or systemicinfections which are caused by pathogenic fungi in mammals.

Ergosterol is the principal membrane sterol of fungi. It is structurallysimilar to its animal counterpart, cholesterol, except that ergosterolhas a methyl group and two double bonds not present in cholesterol. Inyeast, ergosterol affects membrane fluidity and permeability and playsan essential role in the yeast cell cycle. Yeast cells can take upcholesterol and decrease their requirement for ergosterol to very lowlevels, but cholesterol alone cannot completely substitute forergosterol (Gaber et al. (1989) Mol. Cell. Biol. 9:3447–3456). Thoughthe biosynthesis of ergosterol in fungi involves steps distinct fromcholesterol biosynthesis in animals, sterol biosynthesis in differentorganisms share many common steps. Implicated in sterol biosynthesis isat least one cytochrome P450. The term “cytochrome P450” is a trivialname for a class of cytochromes that includes a number of heme proteinsexhibiting a characteristic absorption maximum at 450 nm when combinedwith CO in the reduced state (‘P’ denotes pigment; hence, the name).These cytochromes occur in most animal tissues, plants andmicroorganisms and catalyze the monooxygenation of a vast variety ofhydrophobic substances, including lipophilic endogenous compounds andxenobiotics, serving as oxygenating catalysts in the presence of one ormore electrontransfer proteins or redox enzymes.

In certain embodiments, the test extracts are screened for sterolbiosynthesis inhibitors of potential use as fungicides orantihypercholesterolemic agents identifies agents by the induction oflanosterol 14-?-demethylase, an enzyme in the biosynthetic pathway ofergosterol and cholesterol, in cultures containing the agents. Testsamples which inhibit ergosterol biosynthesis in this system inducelanosterol 14-?-demethylase activity in the culture. In one screeningtest, test samples are incubated in a culture of a Saccharomycescerevisiae strain sensitive to ergosterol biosynthesis and containing agene fusion of a lanosterol 14-?-demethylase clone with a gene forbacterial β-galactosidase. After incubation of the culture, an increasein lancsterol 14-α-demethylase activity is determined indirectly bymeasuring β-galactosidase activity. The culture media contains achromogenic substrate of β-galactosidase such asorthonitrophenyl-β-D-galactoside or5-bromo-4-chloro-3-indoyl-β-D-galactoside, so that active samples areidentified by the production of colored product. For comparisonpurposes, screening tests may employ a lanosterol 14-α-demethylaseinhibitor such as dinaconazole as a positive control.

Anti-viral antibiotics can be identified by screening for inhibitors ofvirus-specific enzymes, such as retroviral reverse transcriptases. Othervirus-specific processes, such as viral uncoating, viral receptorbinding, and cell fusion (e.g., syncytium formation caused by HIV) canalso be targeted in the screening methods of the invention.

The antiviral properties of the compounds may be determined in an assaywhich utilizes the unique properties of the virus. For instance, theinfluenza virus is a negative strand virus with a segmented genome. Thesynthesis of viral mRNA is accomplished by a virally-encodedtranscription complex. Influenza virus is unique in that it requirescapped and methylated palmers which are obtained from host cell RNApolymerase H transcripts to initiate mRNA synthesis. An in vitroinfluenza transcription assay was established to detect agents that maybe present in natural product extracts that are capable of inhibitingthe transcription apparatus of the influenza virus.

U.S. Pat. No. 5,624,928 describes an exemplary assay for detectinginhibitors of the transcription apparatus of the influenza virus whichare required to initiate viral mRNA (messenger RNA) synthesis. Briefly,to each well of a 96-well microtiter plate is added a stock mix of thevirus, the test sample, labeled nucleotides, and water. Ten microlitersof primer (alfalfa mosaic virus (ALMV) RNA at 0.015 mu g/ml) is alsoadded to the wells. The plates are gently mixed on a shaker for 30seconds and then incubated for 60 minutes in a 31 o C. water bath.

At the end of this period, the plates are removed from the water bath,placed on a bed of ice and the reaction stopped with (i) sterilesaturated sodium pyrophosphate solution containing 0.5 mg/ml RNase-freetRNA and (ii) ice-cold 40% TCA, and the plates allowed to stand on icefor 15 minutes. The samples are then collected, using a cell harvester,washed twice with 5% TCA, then twice with 95% ethanol and thentransferred to sealing bags. The incorporation of the labelednucleotides into a reverse transcript of the ALMV RNA is detected.

Anti-Tumor Activities

To identify anti-tumor activities, cultured tumor cell lines or culturedtumors can be contacted with culture extracts, or by co-culturing withthe host cells, and effects on cell growth and viability monitored.Another approach involves screening for products from the host cellswhich induce differentiation of tumor cells, e.g., which causes thesecells to lose their tumorigenicity (Franco et al., (1991) Crit. Rev. inBiotech. 11:193–276). An in vitro disease oriented screening program canutilize a large panel of human tumor cell lines grown initially in vitroand assessed for cytotoxicity by the MTT assay (Carmichael et al. (1987)Cancer Res 47:936–42) and subsequently the sulforhodamine B proteinassay (Skehan et al. (1991) Eur J Cancer 27:1162–8). The aim of thisscreen is to select test extracts exhibiting selective activity againstdifferent histological tumor types.

Enzymes can also be used as targets for identifying anti-tumoractivities. Enzymes that have been successfully employed as targets inthe search for anti-tumor agents include protein tyrosine kinases, whichare components of signal transduction pathways regulated by a number ofoncogenes, phosphatidylinositol kinase, spermidine synthase, andtopoisomerases. As the differences between tumor and non-tumor cellsbecome more apparent, tumor cell-specific targets can be used in thescreens in order to identify activities that are not toxic to thepatient.

Extracts that exhibit anti-tumor activities in biochemical and cellculture assays can be tested further in appropriate animal modelsystems.

Immunosuppressive Activities

Immunosuppressive activities can be identified using a number ofstandard methods in the art, including the mixed lymphocyte reaction,which measures lymphocyte proliferation (Goto et al., (1982) J.Antibiot. 35:1286), and screens for macrophage activation (Tanida et al.(1989) J. Antibiot. 42:1619). Inhibitors of T cell activation can beidentified by growing cultured T cells in the presence of the candidateextract, crosslinking with activating agents, such as antibodies to CD3and CD4 surface molecules and a secondary antibody, which normallyactivate T cells, and determining the level of T cell activation. T cellactivation can be quantified by, e.g., a bioassay in which IL-2production is measured by applying the T cell culture supernatant toCTLL-20 cells, which require IL-2 to live (Sleckman et al., (1987)Nature 328:351).

The cellular immune response involves a very complex set of interactionsbetween antigens, T cells, B cells, macrophages, and numerous factors,such as cytokines, which are released by the cells during the course ofthe interactions. In one embodiment, the test extracts can be tested foreffect on T cell activation. While specificity of the T cell response isdetermined by antigen-specific binding to the T cell antigen receptor(TCR), binding to at least one secondary receptor is also necessary foractivation. One such secondary receptor is CD28 which, upon stimulation,induces the activity of nuclear proteins which can increase theproduction of interleukin-2 and possibly other cytokines by binding toan enhancer region associated with the cytokine genes. Immunosuppressivedrugs which act by suppression of the CD28 pathway may have a number ofadvantages over drugs which act through other mechanisms. Thus,according to the present invention, screening assays forimmunosuppressive compositions can comprise exposing cultured T cells totest extract, where the T cells produce an observable signal as a resultof normal CD28 stimulation. The T cells are cultured under conditionswhich will, in the absence of effective CD28 stimulation, produce theobservable signal, generally requiring the presence of substances whichresult in stimulation of both CD28 and the T cell receptor (TCR). Theassay can thus identify test extracts that at least partially suppressthe stimulation of CD28, thus resulting in a decrease in the observablesignal.

T cells used in the screening assays of the present invention can beobtained from T cell lines which have been modified to stablyincorporate a CD28 enhancer region in reading frame with a reporter geneso that exposure of the cells to conditions selected to induce the CD28receptor will result in expression of the reporter gene. The T celllines may be derived by modifying previously established human or mouseT cell lines and hybridomas, where the starting cell lines andhybridomas are capable of expressing certain cytokine gene(s), asdiscussed below.

A variety of cell lines suitable for modification according to thepresent invention are available from public depositories, such as theAmerican Type Culture Collection (A.T.C.C.), Rockville, Maryland.Exemplary cell lines include Jurkat or HUT-28, human leukemic T celllines; EL-4, a mouse T cell line; BW5147, a mouse cell line; 2B4, amouse hybridoma cell line; and human or mouse T cell clones.

The CD28 enhancer region may be derived from the 5′ flanking region of acytokine gene, where the cytokine gene selected should be one which isnormally expressed in the cell line being modified. The enhancer regionwill include at least that portion of the 5′ flanking region which isbound by the CD28 nuclear protein which is produced as a result ofstimulation of the CD28 receptor, as described below, Suitable enhancerregions may be obtained from such genes as the IL-2 gene, the GM-CSFgene, the IL-3 gene, the G-CSF gene, or the γ-IFN gene.

Extracts found to possess immunosuppressive activity in the cell cultureassays can be further tested in animal model systems. An extractcontaining a candidate compound, or a purified or semi-purified fractionthereof, is administered to an immunocompetent animal, for example, amouse which has a non-MHC matched skin graft, and the effect of thecompound on, e.g., T cell or macrophage activation is determined bymonitoring the immune response of the mouse.

As mentioned above, preferable screening assays are designed to identifybiological activities directed specifically against the target cell,e.g., an infectious pathogen or a tumor cell, and not cells of the hostorganism, in order to decrease the likelihood of toxicity problems.Especially in cases where the potential therapeutic biological activityis directed against a process or structure which may be similar in thetarget cell and the host, it is critical to determine the relationshipbetween the effectiveness and the toxicity of the treatment. This can bedetermined by standard methods using both cell culture assays and animalmodel systems (The Pharmacological Basis of Therapeutics, eds. Goodmanand Gilman, MacMillan Publishing, New York, 1980, pp. 28–39, and1602–1614).

Lipid Biosynthesis

The subject method can also be used to identify genes involved in lipidbiosynthesis, as well as novel lipids produced by the products of thesegenes. To illustrate, surface-exposed unusual lipids containingphthiocerol and phenolphthiocerol are found only in the cell wall ofslow-growing pathogenic mycobacteria and are thought to play importantroles in host-pathogen interaction. The enzymology and moleculargenetics of biosynthesis of phthiocerol and phenolphthiocerol areunknown; though it has been postulated that a set of multifunctionalenzymes are involved in their synthesis, and that these genes areclustered on the bacterial genome. Azad et al. (1997) J Biol Chem 272:16741–5.

Polysacchride Biosynthesis

Yet another class of molecules which can be produced by the chimerichost cells are include novel polysaccharides. For instance, alginate isan unbranched polysaccharide composed of the two sugar residuesbeta-D-mannuronic acid (M) and alpha-L-guluronic acid (G). The M/G ratioand sequence distribution in alginates vary and are of both biologicaland commercial significance. As with the PKS and lipid biosyntheticpathways, the genes involved in alignate biosynthesis are also believedto be localized in clusters, and hence are likely to be isolatable inlarge part in single genomic clones.

Modulators of Extracellular Factors

In one embodiment, the test extracts can be assayed for their ability toalter the bioactivity of an extracellular protein, lipid, carbohydrateor the like. For instance, the assay can be disposed to identifyinhibitors of blood coagulation factors, thrombolytic factors, orenzymes aberrantly upregulated in diseases states, such as superoxidedismutase or the like.

Ligands for Cell Surface Receptors.

In another embodiment, the subject method can be use to identify ligandsfor cell surface receptor protein or ion channel, e.g., proteins whichinteract with an extracellular molecule (i.e. hormone, growth factor,peptide, ion) to modulate a signal in the cell. Exemplary receptorsinclude: a receptor tyrosine kinase, e.g., an EPH receptor; an ionchannel; a cytokine receptor; an multisubunit immune recognitionreceptor; a chemokine receptor; a growth factor receptor; or a G-proteincoupled receptor, such as a chemoattracttractant peptide receptor, aneuropeptide receptor, a light receptor, a neurotransmitter receptor, ora polypeptide hormone receptor. In addition, the subject assay isamenable to identifying ligands for an orphan receptor, i.e., a receptorwith no known ligand, regardless of the class of receptors to which itbelongs.

In certain embodiments, the receptor is a G protein coupled receptors,such as α1A-adrenergic receptor, α1B-adrenergic receptor, α2-adrenergicreceptor, α2B-adrenergic receptor, β1-adrenergic receptor, β2-adrenergicreceptor, β3-adrenergic receptor, m1 acetylcholine receptor (AChR), m2AChR, m3 ACHR, m4 AChR, m5 AChR, D1 dopamine receptor, D2 dopaminereceptor, D3 dopamine receptor, D4 dopamine receptor, D5 dopaminereceptor, A1 adenosine receptor, A2b adenosine receptor, 5-HT1areceptor, 5-HT1b receptor, 5HT1-like receptor, 5-HT1d receptor,5HT1d-like receptor, 5HT1d beta receptor, substance K (neurokinin A)receptor, fMLP receptor, fMLP-like receptor, angiotensin II type 1receptor, endothelin ETA receptor, endothelin ETB receptor, thrombinreceptor, growth hormone-releasing hormone (GHRH) receptor, vasoactiveintestinal peptide receptor, oxytocin receptor, somatostatin SSTR1 andSSTR2, SSTR3, cannabinoid receptor, follicle stimulating hormone (FSH)receptor, leutropin (LH/HCG) receptor, thyroid stimulating hormone (TSH)receptor, thromboxane A2 receptor, platelet-activating factor (PAF)receptor, C5a anaphylatoxin receptor, Interleukin 8 (IL-8) IL-8RA,IL-8RB, Delta Opioid receptor, Kappa Opioid receptor, mip-1/RANTESreceptor, Rhodopsin, Red opsin, Green opsin, Blue opsin, metabotropicglutamate mGluR1–6, histamine H2 receptor, ATP receptor, neuropeptide Yreceptor, amyloid protein precursor receptor, insulin-like growth factorII receptor, bradykinin receptor, gonadotropin-releasing hormonereceptor, cholecystokinin receptor, melanocyte stimulating hormonereceptor receptor, antidiuretic hormone receptor, glucagon receptor, andadrenocorticotropic hormone II receptor.

In other embodiments, the receptor is a receptor tyrosine kinase, e.g.,an EPH receptor such as eph, elk, eck, sek, mek4, hek, hek2, eek, erk,tyro1, tyro4, tyro5, tyro6, tyro11, cek4, cek5, cek6, cek7, cek8, cek9,cek10, bsk, rtk1, rtk2, rtk3, myk1, myk2, ehk1, ehk2, pagliaccio, htk,erk and nuk receptors.

The modulation of cell surface proteins can also include effecting thebioactivity of the adherin proteins, e.g., cadherins, integrins and thelike.

In certain embodiments the subject assays measure the production ofsecond messengers to determine changes in ligand engagement by thereceptor. A “second messenger” is defined as an intermediate compoundwhose concentration, either intercellularly or within the surroundingcell membrane, is raised or lowered as a consequence of the activity ofan effector protein. Some examples of second messengers include cyclicadenosine monophosphate (cAMP), phosphotidyl inositols (PI), such asinositol triphosphate (IP3), diacylglycerol (DAG), calcium (Ca++) andarachidonic acid derivatives. In preferred embodiments, changes in GTPhydrolysis, calcium mobilization, or phospholipid hydrolysis can bemeasured. In other embodiments, the test cells contain a reporter genewhich is sensitive to signalling by the target receptor.

Modulators of Intracellular Signalling.

Still another class of molecules which can be identified in the assay ofthe present invention are those which modulate intracellular signalling,e.g., by inhibiting or potentiating protein-protein (intermolecular orintramolecular interactions), protein-DNA, protein-lipid or protein-2ndmessanger interactions, inhibiting or potentiating intracellularenzymes, or inhibiting or potentiating ion channel passivity, and thelike. As described above, the test extract can be sampled with purifiedor semi-purified components, lysates, whole cells or any otherconvenient way of contacting the products of the recombinant host cellwith the intended target in a manner which permits generation of adetectable signal. That signal may be, for instance, a change in the acell's phenotype, rate of proliferation or survival, transcription of areporter gene, changes in 2_(nd) messanger levels, a change in anenzymes activity towards a detectable substrate (or one which produces adetectable product), a change in the amount or characteristics ofprotein complexes or the localization of a protein, e.g., within variouscellular compartments. To further illustrate, the detection step of theinstant assays can be derived to identify products of the recombinanthost cell that, for illustration, modulate a protein kinase (e.g.,serine/threonine kinase, tyrosine kinase), a protein phosphatase (e.g.,serine/theronine phosphatase, tyrosine phosphatase), interactionsmediated by SH2 domains (e.g., with phosphotyrosine residues),interactions mediated by SH3 domains, interactions mediated by leucinezipper domains, phosphatidyl inositol kinases, adenyl cyclases,interactions involving G proteins (e.g., with a G protein coupledreceptor, between the α subunit with β/γ dimer, or downstream signaltransduction proteins), phospholipases, phosphodiesterases, interactionsbetween DNA binding proteins and DNA, and ion flux through ion channels.The interactions can occur between components of the same cellcompartment, as between two intracellular proteins, or differentcompartments, such as between a cell surface receptor and anintracellular signal transduction protein.

Selective Natural Products

In one embodiment, the assay can be used to identify novel polyketides.Polyketides are naturally-occurring compounds, most often produced bymicroorganisms such as fungi and the filamentous bacteria (theactinomycetes). The route by which these compounds are formed is one ofthe most widespread in nature. It is responsible for a vast array ofnatural products with structures varying from simple aromatic compoundslike 6-methylsalicylic acid (6-MSA) to the gigantic polycyclic ethermaitotoxin, whose molecular weight of 3422Da makes it the largest knownsecondary metabolite. Apart from microorganisms, polyketides are alsoisolated from a wide range of marine organisms (for example, brevitoxin)and higher plants (flavonoids). Many other metabolites containpolyketide-derived moieties as part of a larger structure from anotherbiosynthetic origin, for example, the unusual amino acids, such as4-[2-butenyl]-4-methyl-L-threonine (Bmt) found in cyclosporin, and algalpeptide toxins and meroterpenoids such as tetrahydrocannabinol.

In addition to their wide occurrence and structural diversity,polyketides display a huge range of biological activities. These includeantibiotics (for example tetracyclines and erythromycin), anti-canceragents (daunomycin and dynemycin A), antifungals (griseofulvin andstrobilurins), antiparasitics (avermectin and monensin),immunosuppressive agents (FK506 and rapamycin), and cholesterol-loweringagents (lovastatin and squalestatins). Thus they have long been ofinterest to scientists from many disciplines, including natural productchemists, microbiologists and pharmacologists. Many of the challengingsynthetic targets currently being worked on by organic chemists arepolyketides.

Despite their enormous structural variety, all of the polyketides arerelated by their common biosynthetic origins (O'Hagan et al. (1995) Nat.Prod. Rep., 12:1). They are derived from highly functionalised carbonchains whose assemblies are controlled by multifunctional enzymecomplexes called polyketide synthases. Like the closely related fattyacid synthases, polyketide synthases catalyse a repetitious sequence ofdecarboxylative condensation reactions between simple acyl thioestersand malonate. Each condensation is followed by a cycle of modifyingreactions: ketoreduction, dehydration and enoyl reduction.

Several individual enzymes are needed during the assembly of a fattyacid or a polyketide. These enzymes—ketosynthase, ketoreductase,dehydratase, enoyl reductase—carry out the main chemical transformationsin the assembly sequence. In addition, an acyl carrier protein, acyl andmalonyl transferases and thioesterases are needed to load substrates andremove products. For aromatic polyketides, the participation of one ormore cyclases is also essential. Genetic analysis of fungal andbacterial polyketide synthases has revealed that they come in a numberof distinct forms, and the current state of knowledge of these will besummarised in turn.

In bacterial systems, the polyketide synthases responsible for thebiosynthesis of aromatic polyketide antibiotics are analogous tobacterial and plant type II fatty acid synthases. Here the synthase ismade up of a functioning complex of essentially monofunctional proteins.In both the fungal type I and bacterial type II systems, it is importantto emphasise that it is the same enzymes that are used repetitively ineach cycle of chain elongation and modification. All of the genesnecessary for the biosynthesis of the polyketide antibiotic actinorhodinin Streptomyces coelicolor were found together, ‘clustered’ on the samestretch of genomic DNA. This enables, by the subject method, all thebiosynthetic genes to be readily isolated on a single genomic clone.From this and subsequent work, it has become clear that assembly andcyclisation of the intermediates in the biosynthesis of aromaticpolyketides in Streptomycetes usually requires up to six individual genesequences (referred to as open reading frames or ORFs) from therespective polyketide gene clusters. The remaining genes in the clustersare involved in the initiation and control of polyketide synthesis andthe post assembly reactions which further elaborate the initialpolyketide products to give the final observed structures.

Thus, DNA fragments containing the whole or part of the gene cluster canbe introduced into a host cell, such as a Streptomycetes. In preferredembodiments, all or a portion of the corresponding gene cluster in thehost cell can be inactivated/deleted. In the case where a chimericpathway is generated in the host cell, a “hybrid antibiotic” can beproduced, e.g., by the concerted genes from different, albeit related,biosynthetic pathways. Such compounds can be detected in, for example,by the assays described above for identifying antibiotic agents, e.g.,by biological, biochemical or chemical means.

Another class of small molecule natural products which can be obtainedby the subject method are the macrocyclic lactones. This group ofcompounds shares the presence of a large lactone ring with various ringsubstituents. They can be further classified into subgroups, dependingon the ring size and other characteristics. The macrolides, for example,contain 12-, 14-, 16-, or 17-membered lactone rings glycosidicallylinked to one or more aminosugars and/or deoxysugars. They areinhibitors of protein synthesis, and are particularly effective againstgram-positive bacteria. Erythromycin A, a well-studied macrolideproduced by Saccharopolyspora erythraea, consists of a 14-memberedlactone ring linked to two deoxy sugars. Many of the biosynthetic geneshave been cloned from S. erythraea, all of which have been locatedwithin a 60 kb segment of that organisms genome; thus there is areasonable prospect for isolating corresponding gene clusters.

Still another class of molecules which can be developed by the subjectmethod are derivatives of quinones. Quinones are aromatic compounds withtwo carbonyl groups on a fully unsaturated ring. The compounds can bebroadly classified into subgroups according to the number of aromaticrings present, i.e., benzoquinones, napthoquinones, etc. A well studiedgroup is the tetracyclines, which contain a napthacene ring withdifferent substituents. Tetracyclines are protein synthesis inhibitorsand are effective against both gram-positive and gram-negative bacteria,as well as rickettsias, mycoplasma, and spirochetes. The aromatic ringsin the tetracyclines are derived from polyketide molecules. Genesinvolved in the biosynthesis of oxytetracycline (produced byStreptomyces rimosus) have been cloned and expressed in Streptomyceslividans (Binnie et al. (1989) J. Bacteriol. 171:887–895). The PKS genesshare homology with those for actinorhodin and therefore encode type II(monofunctional) PKS proteins (Hopewood et al. (1990) Ann. Rev. Genet.24:37–66).

Derivatives of several other types of small molecule products are alsolikely to be identified by the subject method. One of these is theantibiotic 2-hexyl-5-propylresorcinol which is produced by certainstrains of Pseudomonas. It was first isolated from the Pseudomonasstrain B-9004 (Kanda et al. (1975) J. Antibiot. 28:935–942) and is adialkyl-substituted derivative of 1,3-dihydroxybenzene. It has beenshown to have antipathogenic activity against Gram-positive bacteria (inparticular Clavibacter sp.), mycobacteria, and fungi. Another class arethe methoxyacrylates, such as strobilurin B. Strobilurin B is producedby Basidiomycetes and has a broad spectrum of fungicidal activity (Ankeet al. (1977) Journal of Antibiotics (Tokyo) 30:806–810). In particular,strobilurin B is produced by the fungus Bolinia lutea. Strobilurin Bappears to have antifungal activity as a result of its ability toinhibit cytochrome-b dependent electron transport thereby inhibitingrespiration (Becker et al. (1981) FEBS Letters 132:329–333.

Bioremediation

In yet another embodiment, the subject method can be used to isolate agene, or set of genes, which produce enzymes useful in bioremediationprocesses, e.g., degradation of pesticides, toxic waste, oil, grease. Inone aspect, the genomic DNA can be cloned from microorganisms originallypresent in a polluted region. In this case, the degrading ability of themicroorganisms which have naturally grown by utilizing the pollutants asan energy source is extended, and the extended degrading activity isenhanced. In any event, the detection step of the assay can be generatedwith a purfied form of the hazardous (otherwise undesirable) material,with a whole environmental sample, or with some semi-purified fractiontherebetween. Utilizing techniques well-known in the art, the ability ofthe gene or gene products to sequester or transform the hazardousmaterial is detected for the test extract.

Nematicidal Agents

In another aspect, the subject methods are useful for the identificationof genes, or biosynthetic products, which can be used to control pestsand, particularly, plant pests. Specifically, the subject method can beused to identify new toxins useful for the control of nematodes. Certaingene isolates and toxins of the subject invention can also be used tocontrol coleopteran pests, including corn rootworm.

Control of nematodes, or coleopterans, using such toxins, or ifappropriate, the cloned genes, can be accomplished by a variety ofmethods known to those skilled in the art. These methods include, forexample, the application of toxin to the pests (or their location), theapplication of recombinant microbes to the pests (or their locations),or, if appropriate, the transformation of plants with genes which encodethe pesticidal toxins

Exemplary assays formats for detecting nematicidal agents include:

a. The Split-pot test: the test detects an anti-nematode agents having arepellent or antifeedant effect on the nematodes and/or a nematicidaleffect. A ‘split-pot’, i.e. a pot divided into two sections by a finemesh material (see Alphey et al (1988) Revue Nematol. 11:399–404), canbe used. Each side is filled with soil. Test extracts are added to thesoil on the side in which a seedling (Petunia) has been planted. To theother side a population of nematodes, e.g., adult Xiphinemadiversicaudatum, are added. After a certain period of time, the twohalves of the pot are separated and the nematodes extracted from thesoil in each half. Root galls are recorded on plants from the treatedsides (antifeedant action). The numbers of live and dead nematodes fromeach half are also counted (nematotoxic effect).

b. Mini-pot test: This test identifies the nematicidal effect of presentin a test extract in soil and its effect on nematode feeding behaviour.Briefly, seedlings (Petunia) are planted in soil. The test extracts,along with a population of nematodes, is added to the soil. Some timelater, the nematodes are extracted and the number of galls induced bynematode feeding on the roots are determined.

Identification of Compounds Responsible for the Biological Activities

The biological activity can be further characterized by purifying thecompound(s) responsible for the activity using standard methods, such asliquid-liquid, liquid-solid, or affinity chromatography with normalphase, reverse-phase, ion-exchange, and gel filtration techniques beingimplemented as needed (Box, (1991) in Discovery and Isolation ofMicrobial Products, Verall, M. S., Ed., Ellis Horwood, Chichester, 1985;Franco et al. (1991) Crit. Rev. in Biotech. 11:193–276). Thepurification can be monitored by co-fractionation of the biologicalactivity, using any of the screening assays described above. Oncepurified, the structure of the compound can be determined using standardmethods, including nuclear magnetic resonance, mass spectroscopy, andX-ray crystallography.

X. Exemplfication

The invention now being generally described, it will be more readilyunderstood by reference to the following examples which are includedmerely for purposes of illustration of certain aspects and embodimentsof the present invention, and are not intended to limit the invention.

EXAMPLE 1

BAC Library of Bacillus cereus DNA

We have constructed a library in pBeloBAC11 from B. cereus strain UW85.Briefly, the UW85 cells were embedded in agarose plugs, and cellularcomponents (other than DNA) were removed by treatment of the plugs withlysozyme, proteinase K and detergent. The DNA was digested in situ, andsize fractionated on a preparative agarose gel in order to isolate sizefragments of interest. The average insert size was 76 Kb and some of theclones contain inserts as large as 170 Kb. The genomic DNA was ligatedinto a BAC vector and used to transform E. coli DH10B, where the insertswe observed to be stable. We screened the library for 10 phenotypes thatare easily tested in culture and found a total of 9 clones that werepositive for 5 of the phenotypes, which were: esculin hydrolysis,ampicillin resistance, lysis of red blood cells, clearing of lecithin,and resistance to zwittermicin A. Since B. cereus is genetically quitedifferent from E. coli based on codon usage and base content (B. cereusis 35% GC whereas E. coli is 50% GC), it is quite encouraging that wefound such a high frequency of gene expression in the B. cereus BAClibrary in E. coli.

These experiments demonstrate that half of the traits tested wereexpressed in an E. coli BAC clone bank of DNA from B. cereus, which isan AT-rich, Gram-positive bacterium, thereby supporting the predictionthat genes from many soil bacteria will be expressed in the BAC library.Second, we have shown that soil DNA is readily cloned in the BAC vector.

EXAMPLE 2

Construction of Library of Soil DNA

We isolated soil DNA by use of the agarose plug system above, and clonedgenomic DNA fragments into pBeloBAC11. Briefly, 5 g of soil was mixedwith 13.5 ml extraction buffer (100 mM Tris-Cl, pH 8; 100 mM EDTA; 100mM sodium phosphate, pH 8; 1.5M NaCl; 1% CTAB). To that suspension wasadded 1.5 mL of 20% SDS. The mixture was freeze-thawed two times for 10minutes, e.g., in a dry ice-ethanol bath, followed by 10 minutes at 60°C., and then incubated for 2 hours at 60° C. The preparation was thencentrifuged at 6000×g for 10 minutes at 27° C. The preparation wastransferred to a new tube and mixed with equal volumes of CHCl₃ andisoamyl alcohol. The aqueous phase was recovered by centrifugation andprecipitated with 0.6 volumes of isopropanol at 27° C. for 1 hour. TheDNA pellet was recovered by centrifugation at 16,000×g for 20 minutes at27° C., washed with 70% ethanol, and resuspended in 500 μl buffer.

DNA was further purified by preparative gel electrophoresis, producing asize range of 25–125 kb. The DNA was digested with HindIII and ligatedto the pBeloBAC11 vector and transfected into E coli. By this method, wehave obtained a total of more than 3,700 clones and we havecharacterized 85 of them in detail. Thusfar, all of the clones screenedcontain inserts, and the inserts have an average size of 28 Kb and arange of 13 to 60 Kb. Considering that we used methods that shear DNAduring isolation, the large size of these inserts suggests that thetarget of 100 Kb inserts is attainable when we use gentler methods. Inan initial screen, we found 8 clones that degrade esculin, an abilitythat the host E. coli strain does not have.

EXAMPLE 3

Detection of Clone Possessing Antibacterial Actvity in Library of SoilDNA

As described above, the subject method can be used for the discovery ofcompounds with antibiotic activity, e.g., against Staphylococcus andEnterococcus, since multidrug resistant strains of various bacteria arebecome more common and are a significant threat to human health.Briefly, 78 plates, each containing 40 clones, were replicated to LBplates with 6.25 μg/ml chloramphenicol. The plates were incubated for 48hours at 37° C. A soft agar overlay was applied to the plates. Thiscontained 1.3 ml LB medium, 0.2 ml of an overnight culture of S aureus,and 1.5 ml of 0.8% agar. The plates were examined after 24 hours ofincubation at 37° C.

One of the clones showed a clear zone around it, indicating that asubstance was produced which inhibited growth of the S aureus cells inthe overlay. The clone was colony purifed, and plasmid DNA was isolatedfrom a culture. The plasmid was electroporated into the same E colistrain background as had been used in the initial screen, and theresulting transformants were retested to confirm that the S aureusinhibitory activity was in fact due to a plasmid-encoded activity.

All of the above-cited references and publications are herebyincorporated by reference.

Equivalents

Those skilled in the art will recognize, or be able to ascertain usingno more than routine experimentation, many equivalents to the specificembodiments of the invention described herein. Such equivalents areintended to be encompassed by the following claims.

1. A method for detecting a compound produced by a biosynthetic pathway,comprising: i) isolating genomic DNA from a source containinguncultivated microorganisms, without an intervening step of culturingcells from the source, by mixing the source with an extraction bufferand lysing the microorganisms to obtain isolated genomic DNA; and thenii) inserting the isolated genomic DNA of step (i) into a replicablevector to obtain clones of the replicable vector comprising the isolatedgenomic DNA; and then iii) introducing the clones of step (ii) into hostcells to obtain a library of host cells, wherein the isolated genomicDNA in at least some of the clones is at least 50 kb; and then iv)culturing the host cells of step (iii) under conditions wherein an openreading frame sequence of the isolated genomic DNA is expressed by thehost cells; and then v) detecting a compound produced by the host cellsas a result of expression of the open reading frame sequence of step(iv).
 2. The method of claim 1, wherein step (v) comprises detecting anon-polymeric compound.
 3. The method of claim 1, wherein step (v)comprises detecting a non-proteinaceous compound.
 4. The method of claim1, wherein step (v) comprises detecting a compound having a molecularweight less than 7500 amu.
 5. The method of claim 1, wherein step (v)comprises detecting a compound that is a direct product of endogenousprecursors for the biosynthetic pathway.
 6. The method of claim 1,wherein in step (iii) the isolated genomic DNA in the clones has anaverage length of at least 75 kb.
 7. The method of claim 6, wherein instep (iii) the isolated genomic DNA in the clones has an average lengthof at least 100 kb.
 8. The method of claim 1, wherein the source ofmicroorganisms in step (i) includes prokaryotes.
 9. The method of claim8, wherein the source of microorganisms in step (i) includes prokaryotesof the Archaea Domain.
 10. The method of claim 9, wherein the source ofmicroorganisms in step (i) includes Archaea cells selected from thegroup consisting of Crenarachaeota, Euryarachaeota, Karachaeota, andcombinations thereof.
 11. The method of claim 1, wherein step (i)comprises isolating genomic DNA from a source selected from the groupconsisting of soil samples, insect intestines, plant rhizospheres,microbial mats, sulfur pools samples, or marine samples.
 12. The methodof claim 1, wherein the source of microorganisms in step (i) includesprokaryotes having low G/G content genomes.
 13. The method of claim 1,wherein the source of uncultivated microorganisms in step (i) includesanaerobes.
 14. The method of claim 1, wherein in step (iii) the libraryof host cells comprises a variegated population of vectors containingdifferent isolated genomic DNA sequences.
 15. The method of claim 14,wherein step (i) comprises isolating genomic DNA having an averagelength of at least 50 kilobases.
 16. The method of claim 14, wherein instep (iii) the library of host cells comprises a variegated populationof vectors including isolated genomic DNA from at least 10 differentmicroorganism species.
 17. The method of claim 1, wherein step (iii)comprises introducing the clones of step (ii) into host cells selectedfrom the group consisting of Acetobacter, Actinomyces, Aerobacter,Agrobacterium, Azotobacter, Bacillus, Bacteroides, Bordetella, Brucella,Chlamydia, Clostridium, Corynebacterium, Erysipelothrix, Escherichia,Francisella, Fusobacterium, Haemophilus, Klebsiella, Lactobacillus,Listeria, Mycobacterium, Myxococcus, Neisseria, Nocardia, Pasteurella,Proteus, Pseudomonas, Rhizobium, Rickettsia, Salmonella, Serratia,Shigella, Spirilla, Spirillum, Staphylococcus, Streptococcus,Streptomyces, Trepanema, Vibrio, and Yersinia.
 18. The method of claim17, wherein step (iii) comprises introducing the clones of step (ii)into host cells selected from the group consisting of Escherichia andStreptomyces.
 19. The method of claim 18, wherein step (iii) comprisesintroducing the clones of step (ii) into an Escherichia coli host cell.20. The method of claim 1, wherein step (ii) comprises inserting theisolated genomic DNA into a low-copy number vector.
 21. The method ofclaim 20, wherein step (ii) comprises inserting the isolated genomic DNAinto a single-copy number vector.
 22. The method of claim 20, whereinstep (ii) comprises inserting the isolated genomic DNA into a BAC or PACvector.
 23. The method of claim 1, wherein step (v) comprises detectingthe compound produced by the host cells as a result of expression of theopen reading frame sequence of step (iv) by an ability of the host cellto induce a biological or biochemical response from a test cell.
 24. Themethod of claim 23, wherein the biological or biochemical responseinduced in the test cell comprises: a phenotypic change, a change ingene transcriptional rates, a change in phosphorylation states, a changein 2nd messenger formation, cell quiescence, or cell death.
 25. Themethod of claim 1, wherein step (v) comprises detecting the compoundproduced by the host cells as a result of expression of the open readingframe sequence of step (iv) by spectrometric detection.
 26. The methodof claim 1, wherein step (v) comprises detecting the compound producedby the host cells as a result of expression of the open reading framesequence of step (iv) by chromatographic detection.
 27. The method ofclaim 1, wherein step (v) comprises detecting the compound produced bythe host cells as a result of expression of the open reading framesequence of step (iv) using a cell-free assay.
 28. The method of claim1, wherein step (v) comprises detecting the compound produced by thehost cells as a result of expression of the open reading frame sequenceof step (iv) by inducing a biological or biochemical response from thehost cell.
 29. A method for detecting a product of a biosyntheticpathway, comprising: i) directly cloning into a replicablevector-genomic DNA isolated from a complex microbial sample without anintervening step of culturing cells from the sample, andsize-fractionated prior to any enzymatic manipulation of the DNA tofacilitate its insertion into the replicable vector, wherein at leastsome of the genomic DNA cloned into the vector is at least 50 kb long;and then ii) transforming the replicable vector of step (i) into hostcells to obtain a library of host cells; and then iii) expressing openreading frame sequence(s) of the genomic DNA contained in the library ofhost cells of step (ii); and then iv) detecting a compound which is theproduct of a biosynthetic pathway that is dependent on expression of atleast one of the opening reading frames of step (iii).
 30. The method ofclaim 1, wherein step (i) comprises lysing the microorganisms andisolating the genomic DNA from the source by further treating the sourcewith a protease and a detergent to yield a mixture, centrifuging themixture to yield a supernatant, extracting the supernatant withchloroform in the absence of phenol to yield a chloroform fractioncontaining the genomic DNA, and precipitating the DNA by addingisopropanol to the chloroform fraction.
 31. The method of claim 30,wherein step (i) further comprises size-fractionating the isolatedgenomic DNA prior to enzymatic manipulation of the DNA for insertioninto the replicable vector of step (ii).
 32. The method of claim 31,wherein step (i) comprises size-fractionating the isolated genomic DNAvia electrophoresis.